Dagon is an ancient real-life deity originally associated with the sea (due to the Hebrews associating his name with their word for fish), but now believed to have been a god of agriculture and fertility. He achieved /tg/-related noteriety when H.P. Lovecraft was inspired by him; one of his first ever published short stories, called "Dagon", presented the deity as a monstrous piscine humanoid who foretold the inevitable destruction of the surface world and the claiming of the surface by the sea. From these roots, Dagon would go on to become a giant-sized demigod ruler of the Deep Ones, a lesser Great Old One in the Cthulu Mythos, from which all of his /tg-related media has arisen.
Whilst most associated with the Call of Cthulhu games and its offshoots, such as Delta Green and Cthulhutech, Lovecraft's Dagon also went on to inspire Gary Gygax, who incorporated him into Dungeons & Dragons canon as a Demon Prince. In this role, he since migrated on to Pathfinder as well.
Dagon in D&D
"There once was an island dead west of the sunset,
A place where the sun's rays danced carefree and bright.
Its shores all a'sparkle with white shining beaches,
And jungles aplenty with game day and night.
They say that the people who dwelt there were happy,
They say that they lived their lives simple and fair,
Yet one day a bitter wind rose from the ocean
And took the poor folk by surprise unaware.
For ruin had risen in thunderous fury,
A ravenous shadow fell over the shore.
And when Father Dagon's black gullet gaped open
The waves ran with blood and the isle was no more."
- – Traditional sea shanty.
Dagon first entered the Dungeons & Dragons in the Monster Manual II for Advanced Dungeons & Dragons 1st edition. Here, he was only mentioned in passing, although it was stated that he ruled an oceanic layer of the Abyss populated by marine demons and various aquatic monsters. He wouldn't be fully fleshed out until the Fiendish Codex dealing with demons was released for Dungeons & Dragons 3rd Edition, and his Demonomicon of Iggwilv article in Dragon Magazine #349.
In 3rd edition, Dagon is stated to be a Demon Prince of the Obyrith type, which means he sits at that intersection between "fiend" and "Lovecraftian monster". An ally of Demogorgon, he rules over the underwater realm of Shadowsea, the 89th layer of the Abyss, which is the underwater portion of the oceanic parts of the 88th layer, Gaping Maw - aka Demogorgon's backyard. Though worshipped as a god by many malign aquatic creatures and races, including Kuo-toa, Dagon is an enigmatic, isolationist deity, who spends most of his time drifting in a near-slumber through this oceanic realm. The aforementioned Demonomicon article provides mechanics for clerics of Dagon, as well as the Thrall of Dagon prestige class.
He resurfaced in Dungeons & Dragons 4th Edition, with his lore mostly unchanged.
|The Demon Princes of Dungeons & Dragons|
|Tanar'ri:||Baphomet - Demogorgon - Doresain - Fraz-Urb'luu - Graz'zt |
Juiblex - Kostchtchie - Lolth - Malcanthet - Miska the Wolf-Spider
Orcus - Sess'innek - Shaktari - Turaglas - Yeenoghu - Zuggtmoy
|Obyrith:||Dagon - Obox-ob - Pale Night - Pazuzu - The Queen of Chaos - Sertrous|
Dagon in Pathfinder
In Pathfinder, Dagon's lore is mostly unchanged. The biggest tweak is that here, he is presented as a once-mindless qlippoth that was transformed into a sapient demon after devouring too many demons and larvae that strayed into his underwater realm. Unlike most qlippoths, Dagon views his new state as a step up and is very happy with the transformation; he technically bears no ill will to his former race, but he won't tolerate their attempts to kill him, either.
For more information, see here: https://pathfinderwiki.com/wiki/Dagon
|The Demon Lords of Pathfinder|
|Demon Lords:||Abraxas - Aldinach - Andirifkhu - Angazhan - Areshkagal - Baphomet - Cyth-V'sug - Dagon |
Deskari - Flauros - Gogunta - Haagenti - Jezelda - Jubilex - Kabriri - Kostchtchie - Lamashtu
Mazmezz - Mestama - Nocticula - Nurgal - Orcus - Pazuzu - Shax - Shivaska - Sifkesh
Socothbenoth - Urxehl - Xoveron - Yhidothrus - Zevgavizeb - Zura
|Nascent Demon Lords:||Daclau-Sar - Izyagna - Kro'akoth - Menxyr - Murnath - Nightripper - Ovonovo - Shamira |
Sithhud - Treerazer