Light the Dragonfires

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.

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 Mehrunes Dagon himself. Martin will inform you that you must proceed to the Temple of One.

Protect Martin at all costs. If you have to act as a meatshield for Martin, then do so. Make sure he doesn't die, which is quite easy if you have to fight Xivilai, Dremora Markynaz and Valkynaz, Daedroths and Storm Atronachs.

The biggest fight occurs in the Temple square itself. There are about 8-10 Daedra, which could be a mixture of the aforementioned Daedra so act as a meatshield for Martin, and it'll all go well. At a certain point in time, Martin will speak to you, once Mehrunes Dagon arrives on site. At this point in time you just want to book it for the Temple of One's door at once and waste no time.

Inside the temple, Martin will show that he is indeed a Septim by turning into a Dragon and defeating Mehrunes Dagon.