You are not supposed to be able to do so, but there are ways. 'Damage Fatigue' and 'Reflect Damage' enchantments can do it, in unpatched versions, Wabbajack can be used to make him killable. By using mods, it is possible to make a weapon capable of killing him - however his model does not have a proper death animation, so collapses and deflates rather comically if you do so.
Defeating Mehrunes Dagon will close the Oblivion Gates, but you could still visit Oblivion afterwards if you do Peryite's daedric quest.
Mehrunes Dagon has 1000HP, but can't be killed under normal circumstances, physical blows simply stagger him. There is a way to kill him though, by hitting him with the Wabbajack obtained from Sheogoroth's deadric quest and then you can kill him because now his statistics will have been replaced with those of a random monster.
folow the main quests and eventually shut down the main portal to oblivion. Not to mention defeating Mehrunes Dagon.
Mehrunes Dagon gets through to the imperial city and Martin Septim sacrifices himself and turns into akatosh (the divine god of time I believe). Akatosh kills Mehrunes Dagon and you get imperial dragon armor for helping out.
The events of Oblivion take place at the very end of the third era - the banishing of Mehrunes Dagon is considered to be the beginning of the fourth era.
The 'Oblivion Crisis' was the failed invasion of Tamriel by Mehrunes Dagon, one of the Daedric princes. His worshippers assassinated the emperor and his heirs, breaking the barrier that prevented a daedra incursion into Mundus (the material plane). The hero in the Oblivion game, located the emperor's last heir, which lead to the defeat of Mehrunes Dagon, though the septim bloodline was now ended, causing a period of turmoil and ending the 3rd era.
Right at the end once you've gone through Mankar Camorans paradise and retrieved the Amulet of Kings. Mehrunes Dagon will attack the Imperial City. Wabajack can destroy him instantly for soem reason...
The Shrine of Mehrunes Dagon is located in the mountains, south of The Pale region. There is a map in the related links below.
Lord Dagon forever reborn in blood and fire from the waters of Oblivion for Lord dagon forever reborn in blood and fire from the waters of Oblivion for Lord Dagon forever reborn in blood and fire from the waters of Oblivion...
You need to be Lv20 to start the 'Pieces of the Past' quest.
The easiest way will be just do the story missions and not to do secondary missions.-----You just do the main quest, you should close the big oblivion gate, get to paradise (as Mankar Cameron and the mythic dawn refer to it) and summon akatosh, the huge dragon god to defeat Mehrunes Dagon (Big demon-looking dude).
In The Elder Scrolls IV and V, the "Prince of Destruction" is know as the infamous Mehrunes Dagon.