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. (more…)