The final quest requires you to escort Martin to the Imperial City and receive the blessing of the Council before he becomes Emperor. Unfortunately, Dagon launches a last ditch attack on the Imperial City in an effort to stop Martin from lighting the Dragonfires. You must keep Martin alive – Convalescence spells are especially helpful – and get Martin from the Council Chambers to the Temple of the One. Dagon is immortal, so you cannot kill him, but, if you deal enough damage in a single attack, then he will be stunned for several seconds. There are also Daedra running around and Martin is wearing only Imperial Robes, so he’s quite vulnerable. Once you run past Dagon and get Martin into the Temple, he will transform himself into the Avatar of Akatosh and slay Mehrunes Dagon. After the final cinematic sequence, the achievement will unlock.