More from Mordenkainen’s Tome of Foes: After the abishai, the next two requests I got were for the oblex, a slime-creature created out of mind flayer experimentation which feeds off humanoids’ memories. On D&D Beyond, Jeremy Crawford recently characterized the oblex as “D&D’s new scariest monster.” Is it? I’m not convinced that it’s the scariest, in terms of the degree of threat it poses—but I would say it’s one of the creepiest.

The main reason I think oblexes (I feel like the plural should be “oblices”) are more creepy than scary is that they don’t need to kill their victims to consume their memories. They can kill their victims, but they don’t need to. Furthermore, it’s not clear that they have any compelling reason to. It’s the memories that power them, not the physical substance of their victims. There’s also a curious choice of wording in the Eat Memories feature that makes me wonder whether an oblex has any good reason to use it more than once per target. But more on that below.

Oblexes/oblices have an unusual ability contour, with peaks in Dexterity, Constitution and Intelligence. Dexterity plus Constitution usually means “skirmisher,” but oozes can’t move fast enough to skirmish. What they really are is quasi-brutes with Dexterity- and Intelligence-based rather than Strength-based attacks and a keen sense of their opponents’ weaknesses.

They also have 60 feet of blindsight, meaning that even when they take an apparently humanoid form, they have eyes in the back of their heads, figuratively speaking, and can zero in on their victims even in absolute darkness. Nighttime suits them, as do subterranean environments.

They’re all Amorphous, able to slip through cracks as narrow as 1 inch wide (the text specifically says “without squeezing,” which I take to mean—based “Squeezing Into a Smaller Space” on page 192 of the Player’s Handbook—that passing through such a crack doesn’t slow them down at all or penalize them on attacks or Dexterity saving throws, nor does it grant opponents advantage on attacks against them). Finally, they all have Aversion to Fire and immunity to several debilitating conditions, four of which are fairly self-evident based on their oozy nature (the charmed condition is the fifth).

The oblex spawn is the simplest and weakest form of the oblex, produced when a greater oblex becomes too full of memories and needs to shed a few. Oblex spawn can’t consume memories themselves yet, so they won’t engage in combat if they can help it. Instead, their goal is to avoid engagement and get someplace safe where they can grow, as quickly as they can manage. If an enemy does engage an oblex spawn in melee, it will fight back, whomping the meddler with a pseudopod—but not if its attacker does fire damage to it. Even a single point of fire damage will prompt the oblex spawn to Dash away, as will being moderately wounded (reduced to 12 hp or fewer). Oblex spawn are intelligent enough to Disengage, but they’re so slow that doing so only hinders them further.

The adult oblex is the basic form, and it’s much tougher than the spawn. It possesses extraordinary Dexterity and Intelligence and very high Constitution and Charisma. The primary benefit of the latter is its skill at Deception, which is to say, impersonation of the people whose memories it’s consumed in the past. “The sharper the mind, the better the meal,” says Mordenkainen’s, so the adult oblex zeroes in on the player character or non-player character with the highest Intelligence in the area. It’s up to you, as the dungeon master, to decide whether it needs to observe the PCs and NPCs for a while to pick out the most intelligent among them or whether it can just sense the rich, delicious complexity in their brainwave patterns.

Actually, the ideal victim is a PC or NPC with high Intelligence and low Wisdom. Given a choice between, say, a character with Intelligence 18 and Wisdom 14 and one with Intelligence 16 and Wisdom 10, an adult oblex will weigh the risk against the reward and may prefer the latter. Figure that the oblex wants a solid two-thirds chance of success and won’t settle for less than 50/50: a target with a +0 or lower Wisdom saving throw modifier is ideal, and one with a +5 or higher modifier is out of the question. On the flip side, a target must have an Intelligence of at least, say, 14 to be of any interest to the oblex at all.

The oblex is like Sun Zi: what it wants is not to fight but to win without fighting. To this end, it has the Sulfurous Impersonation feature, plus four magic spells in its repertoire:

  • A boosted charm person, affecting up to four five (duh) targets at once.
  • Color spray, primarily useful for covering the oblex’s escape if and when it decides it needs to flee. Because of its area of effect, a 15-foot-cone, the oblex can only reasonably expect it to affect two enemies, with hit points totaling 33 or fewer. So we’re talking, like, level 3 PCs here, or maybe a single PC of level 6 or 7—and probably not the one most able to hit the oblex with something sharp or heavy.
  • Detect thoughts, useful for confirming whether the target whom the oblex thinks will make the finest meal really will. The presence of this spell in the oblex’s repertoire may incline you against simply letting it know your PCs’ stats automatically; I think it would do so for me.
  • Hold person, also boosted to affect two targets at once. Good to use against the dumb but dangerous ones whose memories the oblex isn’t interested in. Requires concentration.

So the oblex’s modus operandi is shaping up something like this: First, it uses Sulfurous Impersonation to present itself as a reasonable, not-at-all-suspicious person for the PCs to interact with (or two, or five), relying on its proficiency in Deception to maintain the charade. If Deception fails and the PCs start to get suspicious, it falls back on charm person to keep them in line. From this point on, the oblex has one hour to get what it came for. It also has to stay within 120 feet of its body to maintain its slime-tether, which means that if it spools itself all the way out, it’s limited to its body’s 20-foot movement speed.

This leads us to a point that Mordenkainen’s doesn’t address, and I wish it did: What’s the DC of noticing the oblex’s slime-tether? On the one hand, we can posit that the slime is translucent, making it harder to see. On the other hand, when I hear “slime,” I think “glistening,” and light reflecting off the slime could well make it easier to see. As a kludge, we can declare that the DC is 15, since the DC of all the oblex’s other abilities is 15. But this is going to give any PC with a good passive Perception score, especially one proficient in that ability or with the Observant feat, a strong chance of noticing the slime-tether right away. (Of course, if the oblex puts the whammy on that PC with charm person quickly enough, he or she may look at the slime-tether and say, “This is fine.”)

Anyway, the goal of the oblex’s actions is to get itself close enough to its preferred target—and everyone else far enough from them—for it to surprise him or her with the Eat Memories action. Here’s the next thing about the oblex entry that’s peculiar: The victim becomes memory drained upon failing the saving throw just once. According to the wording of the feature, “When an oblex causes a target to become memory drained, the oblex learns all the languages the target knows and gains all its proficiencies, except for any saving throw proficiencies.” This seems to imply that after one successful Eat Memories action, the oblex’s work is already done. Yet the feature also discusses the effects of multiple uses of the Eat Memories action.

Here’s where I think this must be coming from: the possibility of there being an oblex colony. In other words, if there are several oblices in a location, they’re all going to want a piece of Trawiodol the Uncanny’s recollections. So the second paragraph must, I think, be discussing the effects of being memory drained once each by more than one oblex. Each oblex, however, is sated after using Eat Memories just once.

The “memory drained” condition, as defined by the Eat Memories feature, penalizes the one suffering from it a single die (the size of which is determined by the number of times his or her memory has been drained) on all ability checks and attack rolls, presumably representing moments of flaking out. But while the feature indicates that the memory drain lasts until the target rests or has greater restoration or heal cast upon him or her, it doesn’t say anything about whether the target suffers from general amnesia. I’d take the literal wording of the feature to mean that he or she does not.

OK, so all of that is a roundabout path to the conclusion that the oblex consumes its victim’s memories once, and that once is enough for it. At that point, it would like to leave. But the victim probably isn’t going to just let it, because the act of eating his or her memories terminates the effects of the oblex’s charm person spell on the victim. (Oddly enough, since no damage is inflicted on a successful saving throw, a target who makes his or her save against Eat Memories remains charmed!) Also, what if there’s more than one tasty memory-morsel in the vicinity? The oblex would love to stick around and gobble up that one, too, if possible.

This is where the Multiattack comes in. The oblex has no reason to use its Pseudopod attack against someone whose memories it wants to eat. But once it’s already eaten, the need to defend itself comes to the fore. So we have three possible offensive actions:

  • Eat Memories, by itself, for when the oblex just wants to eat someone’s memories.
  • Pseudopod, by itself, for when the oblex is done eating memories and is defending itself as it retreats.
  • Multiattack, for when the oblex wants to eat one target’s memories while defending itself against another.

Now, here’s a really dirty thought (and yet another possibility that Mordenkainen’s doesn’t explore): The Sulfurous Impersonation feature lets the oblex create a simulacrum as a bonus action. What does it take for an oblex to unmake a simulacrum? The stat block doesn’t say. Can it do this in a bonus action too? If it can, it may not even have to retreat, just let its simulacrum collapse into goo and reel its slime-tether back in. Maybe it’s not that easy, though. Since the stat block doesn’t specify that it can do this, we should probably assume that it can’t. Clarification would be welcome, though, because if the oblex had this choice available, it would surely make use of it.

The problem is that while the hypothetical “Abandon Impersonation” bonus action might be too cheap and easy, without that option, the oblex has no good retreat action and is practically doomed to die. Its armor class is too low for it to Dodge, it’s not fast enough to benefit from Disengage, and as for Dash, it’s both too slow and too likely to be surrounded by enemies who all want to do it in.

For this reason, I think the oblex has to rely on two things in order to retreat effectively: the mousehole it rode in on, and hold person. Consider that if charm person works, the only ones chasing the oblex will be those it harms with Eat Memories (and if it doesn’t work, things may not even get that far). This means it shouldn’t need to cast hold person on more than one or two opponents, and that—plus, in a pinch, color spray—should give it the chance it needs to get back to its source crevice and schlorp through to safety. (Incidentally, this is also what the oblex will do if things go so far wrong early in the process that it never even gets to eat anyone’s memories. As a retreat action, it will prefer Dash but will use Disengage if engaged in melee by three or more enemies.)

The oblex is in much deeper trouble if its enemies find its body. In this instance, if it doesn’t have a crack it can flee through, it’s going to have to lash out with everything it’s got and hope for the best. It will use Multiattack every round, eating the memories of anyone within reach whether or not they’re all that smart, just to inflict the damage. Against opponents within melee range, it uses Eat Memories against more heavily armored foes and Pseudopod against more lightly armored ones. Unfortunately, if its body is attacked with fire, there’s not much it can do about it except try to use its spells to disable whoever’s doing that damage. In that case, the burden of the die roll is on the opponent, so the disadvantage imposed by Aversion to Fire no longer matters.

The elder oblex has the same feature package as the adult oblex, but it can impersonate as many as 13 past victims, it’s better at Eating Memories, and it has more spells in its repertoire. Sensibly, it ditches color spray in favor of confusion, dominate person, fear, hallucinatory terrain and hypnotic pattern, all of which serve the same purpose of disorienting and delaying pursuers. It also has dimension door as a last-ditch method of escape if its enemies find its body, along with telekinesis for amusing poltergeist action. Note, though, that it doesn’t get to boost hold person anymore. Guess it outgrew that.

Sulfurous Impersonation/Deception followed by charm person is still the elder oblex’s best opening play. However, the elder oblex has the means to stay on the scene longer, because its Eat Memories DC is 18 rather than 15. This means it can reasonably expect to succeed against targets with Wisdom save modifiers of +3 or lower, and a save modifier of +8 or higher is necessary to deter it completely. It also has Constitution 21 and a big pool of hit points. It’s going to take what it wants before it goes.

Once it breaks the charmed condition by eating a victim’s memories, it can use its Multiattack to Pseudopod-whomp that victim into submission wile it uses Eat Memories on another. The damage on the elder oblex’s Eat Memories action is ferocious: an average of 44 points, versus just 18 for the adult oblex, and thus much more likely to render a target unconscious from the get-go. So it may not even need to use its Multiattack—but if it does, it does.

What spells it casts after it’s finished snacking and ready to skedaddle depends on how many uncharmed enemies it needs to gain control of in order to make its escape. If it only needs to disable one, hold person is the go-to (or hold monster, if the enemy is a non-humanoid creature, such as a ranger’s beast companion). Against two, it uses dominate person to turn one against the other. Against three clustered up just right, fear; against four or more, hypnotic pattern. If it’s in a natural environment rather than a built environment, it can use hallucinatory terrain to make its enemies think they can’t pursue. (In this company, confusion is the odd spell out, too unreliable and with too small an area of effect to compete. Telekinesis is situationally useful—most of all when the elder oblex’s enemies are succeeding on too many Wisdom saves.) In particular, it uses its spells to deal with pesky ranged attackers, since it can’t reach them to use its attack actions.

When its body is attacked (as opposed to its simulacra), the elder oblex is much more likely to stand its ground, because it can: its spell repertoire can keep enough enemies disabled long enough for it to pick them off one or two at a time. But it’s going to have a hard time against a high-Wisdom party, and it will realize it may be in some trouble when it’s moderately wounded (reduced to 80 hp or fewer). At this point, it will slither away through a crack if it can, dimension door away if it can’t. If it has to retreat through a crack, its AC is good enough that Dodge is a reasonable action for it to take on its way out.

But if it’s truly cornered—no cracks to slip through, no more uses of dimension door—it stands its ground and fights to the death with its Multiattack, even if it’s taken fire damage. Eat Memories may still work, even if it has disadvantage on its Pseudopod attacks, and it does more damage besides.

Next: star spawn.

This article has 15 comments

  1. Another DM Reply

    I’m not sure the hypothetical “abandon simulacrum” action is required. Since the oblex simultaneously occupies both its proper space and the space of its simulacrum, I would argue no movement or action is required for the oblex to retrieve or dismiss its simulacrum. It simply chooses to no longer occupy one of its spaces.

    Regardless, the adult and elder oblex/oblices are both extraordinarily clever. They’ve probably eaten the memories of other clever people and picked up some dirty tricks that way. They are going to plan ahead, and not put themselves into a situation where they will be easily discovered or stuck. The main body will be hidden in the next room, not behind a door but behind an impassable wall with only a tiny crack. If necessary, it’d install a little sliding door to close the crack and cut itself off if that’s what it takes to disappear.

    By the same token, the adventurers are probably not going to spot the slime tether with a DC 15 perception check because the tether is going to run out of sight under carpets, behind curtains, or through whatever cover is available (which the oblex, impersonating a person, will have set up in advance). Hallucinatory terrain to hide the relevant gaps if you’re feeling saucy. Perhaps it tries to meet with people in an office or shop so it can sit or stand behind a table or desk. It’ll make sure there are candles, lamps or incense present to mask the sulphur smell.

    • Another DM Reply

      Ack, I forgot about the aversion to fire. Substitute “perfume” for candles, lamps or incense.

  2. Bob Reply

    From the presence of the “Eat Memories” ability, I would treat the simulacrum as having any learned class features (No warlock or sorcerer spells. Maybe Cleric spells.) and proficiencies.

    From there, I’d do what “Another DM” said and make sure the Oblex has a way to sever the cord quickly (preferably with a free action mechanism) and would make the simulacrum cut and run almost immediately after getting some memories if it’s been damaged much at all. Plus it makes for a great fight for the future when the Oblex is able to throw the entire party in simulacrum form at themselves.

    • Keith Ammann Reply

      Ooh! Making a new simulacrum only takes a bonus action. What if an oblex eats a PC’s memories and then sprouts a new copy of that PC right away?

      • Bob Reply

        Would definitely add to the “scariest monster in DnD” claim. Also, although it’s the same creature, it is called a simulacrum. It wouldn’t be crazy to give the simulacrum it’s own HP bar, or at least divide the Oblex’s HP out among the different simulacrums. Seems odd that I could kill the Oblex’s main form by killing one of it’s tendrils.

        Regardless, having it able to make 2-13 adventurers to defend itself with would be decidedly formidable. (Dungeon idea: meat grinder deliberately designed to find the most capable adventurers it can eat the memories of so that the Oblex creates a “backlog” of different simulacrum options to go with. Great way for the DM’s Curse of constantly creating characters to also be alleviated)

        • Keith Ammann Reply

          The stat block doesn’t say the oblex gets to use class features, though, just proficiencies. So a simulacrum could pick up a weapon and use it with the same proficiency that the PC could, but it couldn’t, say, use Sneak Attack or Rage. Giving it those would definitely be too much.

          • Bob

            From a flavor standpoint, I wouldn’t see why not, though. I think it would use it’s own stats (which would make for some interesting situations where it imitates a barbarian to intimidate, but then can’t attack since the Adult Oblex has a whopping 8 STR), but if it has all my memories and skills, why wouldn’t it know my other skills?

            I’ll admit it’s not necessarily RaW, but I think it follows pretty logically from the fiction, and making the Oblex use its own stats and only one health bar among all the simulacrums is probably a reasonable release valve there, especially if you rule the simulacrums only get their DEX based AC rather than any armor the original was wearing.

          • Another DM

            Can the oblex really create more than one simulacrum at a time? The first time I read it, I thought it would be only one at a time (which matches up with the fact that, e.g., the oblex doesn’t appear to get extra actions or multi-attacks based on extra simulacra). Seems like it doesn’t really have the actions (or the HP) to sustain its own adventuring party.

          • Keith Ammann

            Yes, definitely. This is explicitly described in the Sulfurous Impersonation feature. It can’t create more than one simulacrum per bonus action, but an adult oblex can maintain “1d4 + 1” simulacra at a time, while an elder oblex can maintain “2d6 + 1.”

          • Another DM

            On a third (now) reading, the rules are getting more confusing to me.

            “The oblex can impersonate 1d4+1 different creatures, each one tethered to its body by a strand of slime that can extend up to 120 feet away.”

            So on last read, I read “different” to mean that it was just a limit on the number of known things it could impersonate, but now the next clause where it refers to “each” implies that it can, indeed, have several up at once. The rules also seem to be disturbingly silent about whether there is any limit on the number of copies it can make of the same creature. Perhaps that’s more atmospheric than useful, though, assuming that the Oblex only ever gets one turn’s worth of actions and HP.

            I’m now also thinking about how to resolve spell effects. If I hit two simulacra with the same fireball, how many times does it take damage? If I hit one simulacra with hold monster do both freeze? If one is webbed but not the other are they both restrained? I think the answers are “once,” “yes” and “yes but that seems odd.”

  3. James Reply

    Just to note charm person at 5th level targets five creatures not 4.

  4. Gabriel Reply

    Thank you so much and great job on this one. You gave me an idea to use an elder oblex to work as a ghost in a haunted house, with the ghosts beeing the simulacrums, but I’ll stick with them dissolving after stealing memories

  5. Mitch Reply

    Just a tactical note – when I used the elder oblex, I has it use hallucinatory terrain to raise the apparent level of the ground by 1/2 inch. Not enough to be easily noticed, but plenty to hide its slime tether. Thick grass or something like it is another good option, depending on when and where the encounter happens.

  6. Jbot Reply

    I don’t have the book, but if it can create simulacrum (and that’s all the detail given), then those sim have their own set of actions and act simultaneously with the oblex. They each also split hp with the oblex. The first would have half, the next a quarter, the next an eighth and so on as they are created leaving the body with the same amount as the most recently created SIM.

    It could create them as road blocks as it escapes, having them run up to the nearest pc giving chase and attacking while the body runs. This forces the pc to choose whether to dash past to catch the oblex and take an opportunity attack or kill the sim then pursue.

  7. Mitch Reply

    I think giving the simulacrums their own actions would make this thing incredily powerful. The intention, as far as I can see, is that it is jsut a part of the oblex, like a limb. it can perform any of the oblex’s actions, but that IS the oblex’s action that round. Also, you arent supposed to split the HP between them, it IS the oblex, so any hits on the sim (at the oblex’s AC and saving throws) come off the oblex’s HP pool. No matter how many sims you see, there is oly one creature. similarly, if you ‘hold monster’ a sim successfully, you hold the main body and any other sims as well.

    I ran an ender oblex against 4 PCs at 7th level (though they punch well above their weight) and I gave the oblex a single legendary action. It ‘won’ the encounter (as intended, memory draining the mage then fleeing through a crack). But giving each simulacra its own actions, it could have TPK’d them without breaking a

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