Mehrunes Dagon is a daedric prince whose sphere is destruction, change, revolution, energy, and ambition. In most cultures he is seen as a god of bloodshed, betrayal, and is associated with natural dangers like fire, earthquakes, and floods. He appears as a gigantic, red, 200 foot tall, four-armed humanoid with an axe and talons.
The Mythic Dawn are a diehard cult focused on the ‘eternal dawn’ in which Dagon would come to cleanse the world of all non-followers. They assassinated the Emperor, killed his three known heirs and stole the Amulet of Kings all in Dagon’s name. They claim the mortal realm was Dagon's plane to begin with but other’s think that Dagon was not content with Oblivion where anything he killed did not truly die. The Mythic Dawn used the Mysterium Xarxes which was written by Dagon himself to enter the Savage Garden, where those who died in service to Dagon went.
At the end of first era he managed to enter the mortal realm in Morrowind and destroy the Royal city of Old Mournhold. Almalexia and Sotha Sil came too late to prevent it’s destruction but managed to banish Mehrunes Dagon back to Oblivion. He again briefly entered the mortal realm in Cryrodill during the third era and attacked the Imperial City, destroying the Temple of the One before being pushed back into Oblivion by Akatosh.
His realm is called the Deadlands which are barren, contain oceans of lava, are scattered with islands, and have several structures through out such as towers, walls, elevated pathways, and gates, most of which are in disrepair and non-functional. Many things are on fire, the air is smoky, and corpses hang from poles, overhead arches, and rocks, while heads can be seen impaled on sticks. The interior of buildings resemble slaughterhouses with evidence of human bloodshed. The denizens of the realm and fire towers that dot the landscape will attack any mortal on sight. Only one artifact is associated with Mehrunes Dagon:
Mehrunes' Razor: Instantly kills anything and highly prized by the Dark Brotherhood.
The daedric prince’s enemy is Ebonarm and he is served by dremora, clannfear and scamps.
In Daggerfall he rewarded the player for exterminating a rogue frost daedra.
In Morrowind he asked the player to retrieve Mehrunes' Razor from Alas Ancestral Tomb.
In Oblivion he tried to destroy the Septim bloodline and open the gates to Oblivion causing the crisis. He briefly entered the mortal realm and attacked the Imperial City, destroying the Temple of the One before being pushed back into Oblivion.
In Skyrim he repaired the broken Mehrunes' Razor if the player killed the man who had asked them to retrieve the pieces.