Mehrunes Dagon is the Daedric Prince of Destruction, Change, Revolution, Energy, and Ambition. He is associated with natural dangers like fire, earthquakes, and floods. He is an especially important deity in Morrowind, where he represents its near-inhospitable terrain. In some cultures, though, Dagon is merely a god of bloodshed and betrayal. Dagon's plane of Oblivion is known as Deadlands. As the name suggests, they are barren wastelands, consisting of blackened isles in a sea of lava. The Dremora, Clannfear and Scamps are among his servants.
He definitely assisted Jagar Tharn in his temporary procurement of the Imperial throne, probably because of the decade of deliberate warfare and strife under Tharn. He invaded and seized control of the Battlespire in contravention of the Daedric pact preventing unchecked meddling in mortal affairs by divine beings; the purpose of this was to cripple the capacity of the Imperial College of Battlemages which presented a threat to Tharn's power as Emperor. Mehrunes Dagon was also responsible for the destruction of Mournhold at the end of the First Era and apparently destroyed Ald Sotha, home of House Sotha and Sotha Sil's birthplace. Dagon was also the alleged inventor of the Daedric Crescent.
The Hero of Daggerfall received Mehrunes' Razor as a reward for exterminating a rogue Frost Daedra. In Daggerfall, Mehrunes' enemy is Ebonarm and his summoning day is Warriors Festival.
In Morrowind, the Nerevarine is tasked with the recovery of Mehrunes' Razor. Mehrunes' Razor was once in the possession of Telvanni Magelord Neloth; you can read a humorous story about how he lost it in the book Realizations of Acrobacy.
In Oblivion, the plot centers around Dagon's plot to destroy the Septim bloodline and open gates to Oblivion throughout Cyrodiil to launch an invasion. His motivation for this invasion is unclear; the Mythic Dawn claims that Nirn was Dagon's plane to begin with, but a more plausible explanation is that he is not content with Oblivion, where, unlike the mortal world, nothing he killed would truly die. The Mythic Dawn are diehard Dagon worshipers and work to bring about the coming of Dagon by assassinating the Emperor and three of his heirs. The cult focuses on the eternal dawn on which Dagon shall come to cleanse the world of all non-followers. The Champion of Cyrodiil had to recover the Mysterium Xarxes, a book written by Dagon himself, so that Martin Septim could open a portal to the Paradise of Mankar Camoran, where he was keeping the Amulet of Kings. Dagon himself makes a short but dramatic appearance as a gigantic four-armed humanoid armed with an axe and talons at the climax of the Oblivion Crisis.
In the Unearthing Mehrunes Razor official plug-in for Oblivion, the player is again given the task of recovering Mehrunes Razor.
The name Dagon may be a reference to the literary works of H. P. Lovecraft, specifically The Shadow Over Innsmouth. This story is also the inspiration for the Oblivion quest A Shadow Over Hackdirt. Oblivion has several such Lovecraft references, inspirations and homages. The name may also refer to a Philistine god in the bible named "Dagon".
