Lying deep beneath the surface, they plot their revenge - Aboleth Deep Dive 

Wow - I hadn't even noticed (and I won't share how long it took me to get it...)

I'd say Aboleths - but that's simply because I use them more often in my campaigns. I like Aboleths and their ancient empires, though I think Mind Flayers are probably used at more tables (and it helps they are a lower CR so tables will interact with more of them at lower levels)

They worship Dagon?

@@matthewdunham1689 no

@firesniperbullet2 Followers of Dagon are mostly demons from the Abyss. Aquatic demons such as wastriliths and uzollrus are his favored agents, but almost all demons respect him to some extent. But even outside of the Abyss there are worshipers, as his presence is known through out the multiverse. Main followers on the prime material plane are the kuo-toas, followed by chuuls and humanoids, and then monsters like aboleths, krakens, and sea hags. Certainly other creatures not on this list can also fall for worshiping Dagon. Some of the groups worshiping Dagon consists mostly from clerics and other divine spell casters. They worship Dagon as the one who knows all. Others consists with bards, who respects the power of the song. They are inspired by the tone of the waves, and worships Dagon in hope that they can gain such power that Dagon may know. In either case they live near, on, or in the ocean segregated from most land dwellers' community. They live peacefully out of reach of other cultists, but when they have visitors they are either welcomed or utterly destroyed. They have little use for slaves (and even if they captivate some, they are almost immediately slain as a sacrifice for their lord).

@@firesniperbullet2 wrong

Aboleths have remained pretty cool (even if I have a soft spot for the scientist version of them more so than the psionic version) - though, I'm still confused by the Plane of Water connection