Quest giver: Martin Septim
You must escort Martin Septim to the Elder Council Chambers in order to present his claim to the Septim Throne. However, before you do that, be sure to stock up on healing potions, arrows, repair hammers. Note that if you leave Martin at Cloud Ruler Temple, when you return to escort him, he will be waiting in the stables to the left of the entrance, hidden from view.
Once you are ready, take Martin to see Chancellor Ocato in the Elder Council Chambers in the Imperial Palace. Midway through the conversation, an Imperial Legion soldier will inform you the Imperial City is under attack by Daedra. Martin will inform you that you must proceed to the Temple of the One. A small hit-squad of Daedra burst into the council chamber itself. There are more outside the doors.
Protect Martin at all costs. Make sure he doesn't die, which is quite hard if you have to fight Xivilai, Dremora Markynaz and Dremora Valkynaz, Daedroths and Storm Atronachs. Thankfully you get around seven generic soldiers (two from the Imperial Palace and five from the Bastion), along with Martin, Ocato, and Baurus to help you. If Baurus had died in an earlier quest, Captain Steffan will aid you. If Steffan had died also, you do not receive help from anyone else.
The biggest fight occurs when your small party enters the Temple quarter. Run through the entrance to the Temple District before Martin gets there so that he will stay behind the door. This leaves you to fight all the baddies on your own but stops him being killed. There are swarms of powerful Daedra pouring from several Oblivion gates, which could be a mixture of the aforementioned Daedra - so protect Martin. Note that you cannot enter these gates, and if you kill the Daedra coming out of them, more will respawn. The gates are designed to keep a constant number of enemies on the field. At a certain point in time, Martin will speak to you, once Mehrunes Dagon arrives on site (you'll know Dagon when you see him). You can hurry this process along by running away from Martin directly to the right in the Temple District so that you see Mehrunes Dagon. Otherwise, he moves very slowly, and you'll be needlessly fighting Daedra until he gets to you. Run back to Martin for your conversation.
After the short talk with Martin, run for the Temple of the One's door at once and waste no time getting inside. Once you are inside Martin will appear inside with you. If you are not fast enough to run in, then attempt to hurt Dagon. He cannot be killed, but like all enemies, he can be staggered if he is hit with enough power from one particular attack or spell.
Inside the temple, Martin will show that he is indeed a Septim by turning into a Dragon - the Avatar of Akatosh - and battling with Mehrunes Dagon. Just before martin turns into the avatar of Akatosh, Mehrunes Dagon will rip off the roof of the temple. Then begins the short fight (where suprisingly dagon only manages 2 or 3 hits) ending in Martin killing Mehrunes Dagon and turning into a 100ft high stone staue in the shape of Akatosh. Much like the small statue in the talos plaza district but bigger.