What demons are related to the demon lord Dagon?

Question

I'm planning an underwater adventure for D&D 5e that will contain an underwater portal to Dagon's layer of the Abyss (the 89th layer, called the Shadowsea, apparently). The portal will have demons spilling out of it, but it will be quite small at this point so it should be something the party should be able to stop; hence I want to know what demons would be thematic for being underwater and being associated with Dagon.

D&D 5e doesn't contain much information about Dagon (most of what I know of demon lords has come from running Out of the Abyss, in which Dagon does not feature, or the Monster Manual and Mordenkainen's Tome of Foes, neither of which mention Dagon at all as far I as can tell). The only things I know about Dagon come from the Forgotten Realms Wiki page on Dagon, which is not 5e specific and therefore draws on sources I don't have (I only have 5e materials).

This wiki page mentions:

Dagon was served by aquatic demons, especially wastrilith, [...]

But it doesn't further elaborate on what these other aquatic demons are.

The only appropriate demon I've come across so far is the wastrilith, mentioned above, which appears in 5e in Mordenkainen's Tome of Foes (since it is associated with water, though the book mentions no relation to Dagon specifically, but that Forgotten Realms Wiki link does), but it's CR is quite high for the party level range I'm planning. There is also the hydroloth from the same book, but that's a yugoloth, not a demon, and I'd prefer demons where possible.

What demons would thematically follow Dagon?

Ideally of a lower CR than a wastrilith and ideally from 5e where possible, but neither of those are hard constraints. I'll accept anything from any edition of D&D.

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Related, but about Baphomet: What demons are under the dominion of Baphomet?
Related, but about Juiblex and Zuggtmoy: What demons are related to the demon lords Juiblex and Zuggtmoy?

Unlike that first question, I'm only interested in demons specifically (unless the creature has business existing in the Abyss, such as a yugoloth; I assume that pretty much only demons would be there).

Answer 1

The 1e Monster Manual II states (page 35):

Some layers of the Abyss are devoid of most of the typical inhabitants of other levels. The liquid plane ruled by Dagon, for instance, does have hezrou ... and a form of dretch, but the other inhabitants are octopi, squids, kraken, sea snakes, eels, weird fish, and horrible fish-monsters. In short, use your imagination and creativity to develop these areas.

And the 3e Fiendish Codex I states (page 61):

Dagon is served by all manner of aquatic demons and monsters ... myrmyxicus demons often pay him tribute ... His favored minions are wastriliths ...

Answer 2

3e Demonnomicon (Dragon #349)

Many aquatic races venerate him as a god, including krakens, some chuuls, sea hags, and water naga. The most fanatic of these worshipers are certain ancient tribes of kuo-toa.

Human are not unknown but rather uncommon, most living as pirates, merchants. It did present some options for NPCs (and PCs) to basically gain some HP Lovecraft Fishman themed mutations thru prestige classes. It gave some statistics on some "notable minions" The Voice of Dagon a Fiendish Kuo-Toa Bard and The Uzollru- a unique kind of Obyrith Demon that is a huge Cycloptic lobster

3e Fiend Codex I

Calls out his alliance with Demongorgan. It's mostly the exact same but expanded upon in his Demonnomicon entry. A web enhancement provided an Aspect of Dagon with a lesser CR than the Lord himself. While an addition to the FC I called "The Lost Annals" mentions 'roaming gigantic half-alive shapeless clouds of discolored pollution that stripped the flesh from anything that entered them.'

Both of the above tend to hint that sahuagin and their god Sekolah are generally apposed to Dagon.

Demongorgan & Dagon The sitcom

The adventure path Savage Tide (Dungeon #139-150) is mainly focused on Demongorgan and his schemes, it did delve into Demongorgan & Dagon's relation ship- which was only briefly mentioned in the manual of the planes before. Since the Prince of Demons resides on layer 88 the Gaping Maw, and Dagon's on 89. They're considered "Neighbors" and many portals and "bending" areas leading from 88's Water regions right into the Shadowsea. They have a weird relationship where Demongorgan has believed to have bullied Dagon into a forced-alliance. While Dagon himself merely sees Demongorgan as the next one of many to be Prince of Demons. In exchange for forgotten secrets Dagon borrows minions from the ranks of Demongorgan from time to time. In addition to creating new monsters and demons for all the other Demon lords. Also for one reason or another, a few aboleth revere Father-Dagon.

Obyrith

Dagon is an obyrith, which means all sorts of things depending on which branch of lore you care to follow, and which edition you are looking at. Obyrith demon lords have tended to have a higher concentration of Minor-Obyriths at their beck and call (Uzollru, Myrmyxicus, Wastriliths for examples).

D&D 4th Edition

Monster Manual II

Dispite having a stat entry, doesn't say much else about him.

Gloamnull, City of Rain

4e's secrets of the Elemental Chaos had a neat little entry about a city of Genasi who have turned to worthship their Savior-Demi-God and are now even more emotionally bi-polar than their normal 4e-breathen. They forcibly but friendly attempt to indoctrination other Genasi when possible.

Calls out Zealous Genasi, Canoloths, Gorgon, Foulspawn, Aboleths, and undead as minions in the encounter tables.