Description:
Mehrunes Dagon is possibly the most dangerous of all Daedra Princes. He's associated with destruction of all kinds, be it 'simple' bloodshed or natural hazards such as fires, earthquakes or floods. He is considered one of the "Four Corners of the House of Trouble" by the dunmer temple and Morian Zenas lists him as one of the five Daedra most likely to qualify for the title "evil".
History:
Lord Dagon has probably made a larger number of appearances in person than any other Daedra Prince.
In the year 2920 of the first era, he destroyed the Dunmer capital of Mournhold all by himself, slaying Lord Indoril Brindisi Dorom in the process. He was subsequently defeated and banished back to Oblivion by Almalexia and Sotha Sil, but the city was completely destroyed. A new city was built on the ruins, and a statue was errected to commemorate this event.
In the years 389 and 399 of the third era, Lord Dagon took over the Imperial Battlespire - the training facility and proving ground of the Imperial Guard - using a host of Daedra servents weilding Daedric crescents. He and his forces were defeated by an unknown hero (you, if you played Battlespire) and the Battlespire was destroyed.
The hero was helped by Chimere Graegyn, a man who had previously tried to trick Lord Dagon and paid a horrible price: His home, Caecilly Island, was cast into the void, all its citizens were horribly slain, and Chimere was condemned to live forever among the ruins of his greatest ambition, eventually losing his sanity. Graegyn believed that Mehrunes Dagon acted at the behest of the Imperial Battlemage and arch-traitor Jagar Tharn when invading the Battlespire and that without the unknown hero, the Eternal Champion and Ria Salmane would not have been able to defeat Tharn.
In Daggerfall and Morrowind, Mehrunes Dagon could be summoned to gain Mehrunes' Razor.
Warning:
Although Sotha Sil and Almalexia were able to defeat Mehrunes Dagon, it took them incredible efford at the zenith of their power - long before their connection to the Heart of Lorkhan was cut off - to accomplish this. Judging from the intro visible on the official Elder Scrolls website, Lord Dagon will in all likelyhood be the one to invade Tamriel in TES4: Oblivion. IF you have to face him, do not expect to survive the encounter.
For more information on Mehrunes Dagon, visit The Imperial Library.