From their description, you’d think aboleths were among the bossest of all boss monsters, but in fact, they have a challenge rating of just 10—well within the power of a party of medium-high-level adventurers to take on, assuming they have some way to reach the creatures’ underwater lairs.
The Monster Manual classes them as aberrations, but they don’t originate from some other plane of existence, despite having a connection to the Elemental Plane of Water. Rather, they predate all the gods and intelligent beings of the contemporary material world. They are the Old Ones. To me, it’s cool to think of them as the product of a different, much more ancient path of evolution, like holdovers from the world-sea of the Ordovician Period in our own history, 450 million years ago, and their connection to the Elemental Plane of Water as a way they discovered of perpetuating their existence over the eons.
Aboleths have high Constitution and extraordinary Strength, but it’s their exceptional mental abilities that define them. With their high Wisdom and exceptional Intelligence and Charisma, they’re schemers and manipulators par excellence, with superior situational awareness. Because they can’t be permanently slain, according to the MM flavor text, their self-preservation impulse doesn’t manifest the same way it would in an ordinary mortal creature.
As for their physical abilities, they fit the brute profile most closely, though not perfectly, their Constitution falling somewhat short of their Strength. They have no ranged attack and engage in melee without reluctance, but their preference is for a short and decisive battle, settled by their phenomenal Strength, over a drawn-out one.
Aboleths have proficiency in Constitution, Intelligence and Wisdom saving throws, but not in Dexterity, leaving them vulnerable to damaging area-effect spells. This suggests that they have little concern for spellcasters in general, but an opponent who casts this particular type of spell will become a high-priority target.
Aboleths also have proficiency in History (not sure why—it’s not like they’re generous about sharing their knowledge) and Perception, they have darkvision out to 120 feet, and they can make a Wisdom (Perception) check on another creature’s turn as a legendary action, so it’s hard to get close to one without its noticing. Unless a player character knows Deep Speech, aboleths will communicate only by telepathy—and they’re not interested in communicating with anyone they can’t use to further their own interests. They come to this judgment through their Probing Telepathy ability, which grants them insight into a creature’s desires if that creature answers their telepathic call, and once they deem a creature minion-worthy, they seal the deal with the Enslave feature. More on that later.
For now, let’s talk about the aboleth’s melee capabilities. Its tentacles and tail have a 10-foot reach, and it’s surrounded by a 5-foot-thick Mucous Cloud that has the unsettling effect of infecting other creatures with a pathogen that prevents them from breathing air. Thus, it can easily reach and strike an opponent who can’t easily reach it to strike back. Its tentacles transmit a similar pathogen, one that even more unsettlingly causes a target’s skin to become slimy, translucent and effectively allergic to open air, and its Multiattack comprises three tentacle attacks in a single action. The aboleth’s tail attack, which does more raw damage than a tentacle attack but has no pathogenic effect, isn’t part of its Multiattack, but it is available as a legendary action.
Aboleths have a swimming speed of 40 feet; meanwhile, your PCs, remember, can swim at only half their base movement speeds, unless they’re granted a swimming speed by a spell (such as alter self) or magic item (such as a cloak of the manta ray). Thus, it should be relatively easy for them to pursue and attack any target they choose, although it may take a couple of rounds.
Finally, they have the Enslave ability. Yes, its targets get to repeat their Wisdom saves anytime they take damage. But remember, aboleths are crafty: all your party needs is one weak link, and they’ll zero in on it. Are an aboleth’s opponents able to roam and breathe underwater because of a spell being sustained by a single character? Fine—it will Enslave that character, then command it to drop the spell. Enslave is hardly worth using for purposes so prosaic as making party members fight among themselves. More likely, an aboleth will try to Enslave a character already poisoned by its tentacles, telling him or her that it’s useless to resist: his or her prior life is over, and his or her only future now is as a servitor of the aboleth.
So who are the aboleth’s top-priority melee targets? Elves, who have advantage against being charmed. Spellcasters who throw area-effect spells that require Dex saves. Anyone in possession of a magic item that threatens the aboleth. Anyone capable of inflicting moderate damage (more than 40 hp) to the aboleth in a single turn. High-Wisdom characters, who are better equipped to resist Enslavement—unless it’s moderately injured (reduced to 94 hp or fewer), in which case it switches its focus to low-Wisdom characters, those with Wisdom save modifiers of 0 or less. Depending on how many such characters are within its reach, it may aim all its tentacle attacks at one opponent or divide them up among several.
Aboleths are legendary creatures; I’ve already mentioned two of their legendary actions, Detect and Tail Swipe. It uses Tail Swipe to counterattack on a character’s turn anytime that character tries to flank-attack it, Detect on the turn of any character who tries to sneak up on it. The third legendary action is Psychic Drain, which absorbs hit points from an Enslaved creature. An aboleth uses this legendary action on any turn in which (a) it’s moderately or seriously injured, (b) it has a creature already Enslaved and (c) it has the legendary actions to spare (since Psychic Drain costs two). Note that this damage gives the creature another saving throw to break free of its Enslavement, so the aboleth prefers to use Psychic Drain on unimportant Enslaved creatures—ones it Enslaved opportunistically, as opposed to, say, ones it enslaved because they were buffing their companions.
An aboleth in its lair also has access to lair actions. One is a phantasmal force spell whose only useful purpose is to distract a low-Intelligence opponent from fighting the aboleth itself, presumably while it focuses on more important enemies. One is a “grasping tide” that pulls creatures from land into a pool of water. This is useful in a situation like the “Watcher in the Water” scene in J.R.R. Tolkien’s The Fellowship of the Ring, but it’s not at all useful when a battle is taking place entirely underwater.
The third lair action is the only consistently useful one, requiring all underwater enemies within 90 feet to make a Wisdom save against psychic damage. Unfortunately, while an aboleth isn’t restricted in general from using the same lair action two rounds in a row, it can’t use this one a second time without using one of the others in between. Booooo! The practical upshot is that an aboleth will either use the phantasmal force lair action as a reset button or forgo the use of further lair actions altogether, because the grasping tide lair action is so wholly situational. Because it may never get a second chance to use it, an aboleth will use the “rage water” lair action only when all its opponents are within 90 feet of it.
Because its lair actions aren’t nearly as overwhelmingly advantageous as other legendary creatures’ lair actions are, an aboleth is more willing than they to retreat from its lair in order to preserve its continued physical existence. Having to be reborn on the Elemental Plane of Water is better than dying outright, but it’s still an inconvenience. An aboleth that’s seriously injured (reduced to 54 hp or fewer) will retreat from the engagement, Disengaging (action) if necessary but otherwise Dodging (action) as it moves away, and continuing to use its legendary actions as appropriate.
All this, however, relates strictly to combat. Before combat ever ensues, there’s an opportunity for social interaction with the aboleth, and this should be as weird and scary as you can make it. Remember, the aboleth has a memory going back 450 million years, and its intelligence is as different from a PC’s as a squid’s, a sea urchin’s or a lamprey’s would be. Also, the PCs’ gods annoy it, and at best, it considers them useful idiots. It communicates with them telepathically, knows exactly what they want most, probably finds this either drolly amusing or senseless and absurd, and considers it self-evident that they’d be better off as slimy, aquatic servant-beasts.
Finally, if you want to add some variety to an aboleth encounter, chuuls are an obvious and natural option.