Sibriexes are fiends—demons, to be specific—but there’s also something distinctly aberration-like about them. Partly, it’s their Lovecraftian body-horror appearance; partly, the fact that they move only by floating; and partly, the fact that their mere presence is toxic to living things.
Their ability contour is bizarre: extraordinary Constitution and mental abilities alongside merely average Strength and almost nonexistent Dexterity. From this we can conclude that they’re heavily dependent on magic and make no effort to avoid attacks—“psychic brutes,” if you will. We’ve seen this before in one other monster: the githzerai. Githzerai, however, are highly mobile. The sibriex is a slow-moving juggernaut.
Despite their extraordinary Wisdom and Charisma, sibriexes don’t have much reason to stay and chat, nor do they have proficiency in any social skill that suggests what kind of conversation they might engage in. Therefore, I’d say, a sibriex that weighs the odds and finds itself outmatched simply never bothers to engage. The upshot of this is that a sibriex encounter should always be Deadly (see “Encounter Difficulty,” Dungeon Master’s Guide, page 82); throw in a handful of minions if you need to. When it does engage, it uses its telepathy to tell the player characters what it’s going to do to them, in nasty, dripping detail.
Sibriexes have resistance to physical damage from nonmagical weapons, along with cold, fire and lightning, and they’re fully immune to poison. They also have both Magic Resistance and Legendary Resistance, so they have little to fear from spells that require saving throws. Of the small handful of things that make them the slightest bit nervous, one thing stands out: clerical magic—which often deals radiant or necrotic damage, to which they’re not resistant—delivered directly through weapon or spell attacks. This will make a sibriex sit up and take notice.
Contamination is a passive damage feature that requires a sibriex to be within 30 feet of its foe(s). It turns the ground around it into difficult terrain, making it harder for them to get away. Ideally, somehow, a sibriex would like to be able to drop right on top of one of its enemies. Given its 20-foot movement speed, this is going to be hard to achieve unless its opponents come to it. Once they do, however, even its slow movement is enough for it to stay on top of them. Hold this thought: it’s a piece in a puzzle that we’re still putting together.
The sibriex can cast certain spells at will, and a couple of them work nicely in conjunction with Contamination. Command can compel another creature to approach. Hold monster can pin one in place, possibly long enough for the sibriex to close with it. Charm person, on the other hand, has a range of only 30 feet, and it’s broken as soon as the sibriex’s aura of corruption causes poison damage to it—so the sibriex can use this spell effectively only on opponents who’ve already been exposed to its Contamination, made their saving throws and gained immunity to it.
Three times per day, a sibriex can cast feeblemind, which targets a creature’s Intelligence—a common dump stat for non-wizards in fifth-edition Dungeons and Dragons. Moreover, with its own supernatural Intelligence, it can pick out which of its enemies have low or merely average Intelligence as if it were reading their character sheets. A cleric or paladin is an ideal target for this spell, although with its spell save DC of 21, a sibriex has a two-thirds or better chance of overpowering almost any target’s resistance. Not many PCs have an Intelligence saving throw modifier of +6 or better—and if one does, he or she probably doesn’t have the Wisdom saving throw modifier to resist charm person, command or hold monster. (Casting feeblemind on a fighter, barbarian, rogue or monk would be a waste, though, since the spell’s chief benefit is that it shuts down spellcasting.)
Sibriexes have two melee attacks, Chain and Bite. Chain has much greater reach and also does greater expected damage than Bite, so the only reason to use Bite is if a target has some kind of resistance against physical damage, specifically piercing damage. (I’m wondering whether this is a typo: I’d think the chain would do bludgeoning damage rather than piercing.) Bite would be good, for instance, against a Raging barbarian—and a sibriex would know this. But there aren’t a lot of features or items that confer resistance to physical damage and not other types of damage, and there especially aren’t many that confer resistance to one type of physical damage and not another. So Chain is the rule, and Bite is the exception.
Squirt Bile is a ranged attack, and it’s a saving throw ability, so the onus is on the defender to resist it. In this case, the DC is 20 rather than 21 (presumably because it’s biological, and therefore Constitution-based). It always targets a single creature, so there’s no need for fastidious positioning. Warp Creature is also a ranged feature with a DC 20 saving throw, and it targets up to three creatures. This is the ability that’s really murderous, because of its tremendous range (120 feet, which it shares with Squirt Bile) and because it imposes levels of exhaustion. One or two levels of exhaustion isn’t a big deal, but three or more can be catastrophic. Not only that, a sibriex can use Warp as a legendary action on other creature’s turns.
The fact that Warp Creature, if it reaches full potency, turns another creature into an abyssal wretch—a weak minion demon—screams out that this is the feature the sibriex wants to use more than any other. But only up to a point: just one successful save is enough to render a target immune to any further uses of Warp Creature. So even though it’s got a two-thirds or better chance of success against an enemy with a Con save modifier of +5 or lower, its enemy has to fail that save more than once—ideally, six times in a row. What does it take to accomplish that? At least a 93.5 percent chance that the target will fail each saving throw, which means a Con save modifier of +0 or less. Shoot. That ain’t too promising, is it?
Well, the sibriex knows the odds (Intelligence 25, after all), so while it will initiate combat by using Warp Creature on whichever three player characters have the lowest Con save modifiers—and use its Warp legendary action to accelerate the process—as soon as any given PC makes his or her save and gains immunity, it stops using Warp against that individual. It will try using Warp on everybody present, working its way up the Con save ladder, just for the side benefits of the poisoned and exhausted conditions. But it’s not going to get its hopes up.
ETA: Commenter Adam notes that after the third failure, the target incurs disadvantage on saving throws because of exhaustion, and also that the six failed saves need not be in a row, but can include up to two successes in between. However, the difference isn’t huge. For the sibriex to have a two-thirds chance that the target will fail six saves in a row with disadvantage on the last three, the target must have an 88.5 percent chance of failure on each roll, which means a Con save modifier of +1 or less; and the two allowable successes bring it to just shy of +2. This is enough to create a nonnegligible risk that a single opponent will get perma-warped if battle goes on long enough, but it’s not enough to alter the sibriex’s tactics in any meaningful way.
Once combat is initiated, it will drift inexorably toward its targets, and this is where all the pieces of the puzzle come together, because in order to stop the sibriex, they’ll have to either run up and attack it or lob spells from afar. Spells requiring saving throws, the sibriex is likely to shrug off. Melee attackers will get up in its ugly mug only to discover the contaminating effect of standing next to it—then struggle to get away. Ranged weapon and spell attacks that do any significant amount of damage to it (say, 15 points or more) will be answered with sprays of bile.
The sibriex will use Warp Creature both on its own turn and as a legendary action as long as there exist three or more targets who haven’t made successful saving throws against it yet. When it’s down to two, it uses Warp Creature on its own turn but uses its legendary actions for other things. At zero, it abandons Warp Creature entirely and uses Multiattack if it has one or more targets within reach of its chains, Squirt Bile or a spell otherwise. But an exception to this pattern is if a cleric or paladin lands a ranged weapon or spell attack that does radiant, necrotic or some other type of damage it’s not resistant to, in which case it will cast feeblemind against that enemy at the first available opportunity—including as its action, if it doesn’t have any legendary actions left.
In general, it uses feeblemind against higher-Wisdom, lower-Intelligence opponents; hold monster against lower-Wisdom opponents with Extra Attack or Sneak Attack; command against other lower-Wisdom opponents; and charm person specifically against lower-Wisdom opponents who’ve already made their saving throws against Contamination.
I haven’t talked about dispel magic yet. Cast innately, it’s always cast at its lowest possible level, which is 3rd. But to neutralize spells of 4th level or higher, the sibriex gets a +7 to its ability checks, against a DC from 14 to 19. That gives it a two-thirds chance of nullifying a 4th-level spell and a 50 percent chance of nullifying an 8th-level spell. It can cast dispel magic at will, but it does have to consume either its action or a legendary action to do so.
I’m going to posit that a sibriex only bothers to dispel two types of spells, but that when it encounters one of them, it always tries:
- Continuous spells that directly hurt or impede it which it can’t use a saving throw to overcome. (These are usually going to be environmental control spells, such as wall of fire, or a cheapie lock-on attack like witch bolt.)
- Spells that offer direct protection against its own abilities, such as enhance ability and holy aura.
It’s unusual that PCs will encounter a sibriex on the material plane or anywhere else except the Abyss. If they do, it shows no fear of death—it will re-form in the Abyss if it’s destroyed—and does its thing until the PCs take it out. But in the Abyss, it can be killed, and it will retreat when it’s seriously wounded (reduced to 60 hp or fewer). Its armor class is high enough that even though it’s smart enough to Disengage, it won’t bother; rather, it Dodges as it rises directly into the air, vertically, until it’s out of sight. If pursued by a flying PC, it uses its legendary actions (which it still gets) to spray him or her with bile—or, if it’s at least 40 feet up, to cast either hold monster or dispel magic, causing the PC to fall out of the sky. Oops.