Dungeons and Dragons is full of weird and terrible creatures and few are weirder or more terrible than the Aboleth. Resembling twenty foot cat fish with four tentacles, three vertically arranged eyes on the front of their heads, and a slimy blue green body one could be forgiven for assuming it was merely a bizarre aquatic creature when they are in fact fiendishly intelligent and ancient denizens of the worlds' waterways. Aboleth are the oldest race in mortal realms with a history that goes back millions of years. Their unfathomably ancient nature is compounded by a ancestral memory that grants most Aboleth direct experience of this history. They remember a time before the gods and land dwelling races held any sway. Even putting age and size aside the Aboleth remain terrifying. They possess telepathic powers allowing them to craft illusions and take control of the weak willed and a mutagenic slime that turns those who touch it into hideous fish men.
For all of their powers Aboleth remain a distant threat simply because they are so ill suited to the land. While they can survive outside of water for short periods and crawl slowly they dry out fast. In sufficiently dire circumstances they can secrete a mucus that will harden into a barrier and go into hibernation but this leaves them defenseless. But the single biggest protection humanity has from the Aboleth is their sheer indifference to surface dwellers who are little more than a source of slaves and occasional trinkets from the Aboleth perspective.
Mechanically, Aboleth are lawful evil aberrations with a high challenge rating, putting them on the same footing as Mind Flayers or Beholders. They also share the psychic slaver tendencies with Mind Flayers and a definitely non-anthropoid morphology with beholders. I think of Aboleth as having a weird verisimilitude in that they are sufficiently different from humans to feel truly alien but also have a form that look like it could conceivably exist. Those familiar with the mythos (directly or second hand) will recognize the Lovecraftian influence of ancient monsters in the sea. They fill a narrative niche of the rare, barely understood threat; being distant enough that only sporadic contact occurs with humanity but when it does the outcome can be a fate worse than death.
IRON NODER: TOKYO DRIFT