Kuo-toa
Kuo-Toa are Dungeons & Dragons fish people living in deep seas and underground caverns. They are batshit-insane theocratic slavers... in all fairness not an uncommon aggregation of Underdark traits.
Other traits include being able to feel the presence of invisible or ethereal creatures, and armaments that capture rather than kill. Their character-classes tend to fighter, monk ("Monitor"), and cleric.
Their society is theocratic - archbishop rules, children of archbishop ("hooks") inherit the power after they fight each other to the death. Their goddess is Blibdoolpoolp, who actually has humanoid (mammalian - and how!) characteristics rather than kuo-toa. They were introduced in module D2, living in / around her shrine in the Greyhawk setting. They got a larger city in the second act of Night Below.
Later canon has ruled that they were former slaves of illithids, so suffer PTSD from having their brains scrambled by said slaver octopi. Thus they worship whatever seems the most awe-inspiring for them, apparently what was also awe-inspiring for Gary Gygax: monstergirl boobies.
5th Edition takes a leaf from Terry Pratchett and has them make a thing they believe in come true - this way, they create minor gods from whatever they start worshipping. This was how Blibdoolpoolp came into being: made from a broken statue of a woman and crayfish parts.
Their relations with other Underdark races are understandably contentious. Drow and (more often) aboleth sometimes subjugate their communities. Elsewhere drow just try to wipe them out.
Their Niche
Gary Gygax for his ongoing Depths series needed some sort of exotic morlock as might be found this far down. The lore was already groaning under Too Many Humanoids even by the late 1970s. The lore further had sahuagin, to knock off Lovecraft's Deep Ones; here, the narrative needed something clearly weaker than drow. He'd already whiffed on his first idea, to bring illithids into D1's narrative - he wasn't going to risk that again. So for D2 he pulled a second Lovecraftian trope, that of madness.
This constrained the kuo-toa nature at their very genesis - here are those fishmen as cannot compete with the others, so hide in the caves. The kuo-toa, thereafter, end up pathetic and even slightly ridiculous. Their role as an Underdark race has been as serfs or patsies for the real enemy. They're not even as scary as the kopru in Isle of Dread. (Later, the second Monster Manual will serve up the aboleth; later still, the Deep One niche will be more-or-less effectively met by the skum.)
As Deep-One failures the kuo-toa work well. They are alien, intermittently hostile, and unpredictable. But if PCs can swallow their disgust and distrust, they can find allies here.
Gallery
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Module D2
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Kuo-toa fighting Svirfneblin.
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A kuo-toa breeding pool.
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D1-2 combined module
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1e Fiend Folio
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2e
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3e
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4e
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5e
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