Mehrunes Dagon is worshiped by the Mythic Dawn cult.
He is a very large four armed Daedric prince wearing very little armor. He holds a large hammer or axe in one hand, and has three blades attached to another. His sphere of control is one of chaos and destruction. Whenever a flood, earthquake, volcanic eruption or any other natural disaster occurs, it is believed Mehrunes Dagon has attacked Tamriel without the use of Oblivion Gates.
He is not liked very much by the other Daedric princes, since he has broken several pacts created between them. He invaded and controlled the Battlespire, killing off a large portion of the Imperial College of Battlemages. This was the first pact he broke, to not interfere with the business of mortals.
The second time was mention in Morrowind, since a lot of amateur summonings were going wrong, Sotha Sil called the attention of all the Daedric princes and made them offerings so that they would accept another pact. This pact was to ensure the Daedric princes would not respond to an 'amateur summoning'. Sotha Sil said the Daedric princes should only present themselves when summoned by himself, or other powerful mages. However, a witch summoned Mehrunes Dagon and he responded, thus breaking the pact. Eventually he destroyed all of Mournhold, capital of Morrowind, until he was banished back to his sphere by Sotha Sil.
Ever since then, Mehrunes Dagon has become bitter and is hated (but not openly) by the other Daedric princes.