When you arrive at Cloud Ruler Temple and have talked to Martin, you will then need to escort him to Bruma and to the Temple of Talos. After you have arrived, go to Castle Bruma and speak to the Countess about the plan to open the Great Gate. She will agree and then you will need to escort her to the Temple as well. Make your way to the lone open Oblivion Gate, just outside the city and Martin will give a rousing speech to the gathered soldiers there. There should be a good number of soldiers here if you did the “Allies for Bruma” quest.
Once his speech is finished, Oblivion gates will begin to open and the enemies will start to pour out. Kill those that appear while at the same time keeping Martin alive. Martin does a pretty good job of looking after himself, but if you see him getting surrounded, step in to give him a hand. As you fight, other gates will open, allowing more and more Dremora to enter the battle. Once the final great gate eventually opens, runs through it and then you will not need to worry about keeping Martin alive as he’ll take care of himself while you are in the Oblivion Plane.
Once you enter the gate, make your way forward and enter the tower on the right. Follow the path leading upwards and try to keep moving as quickly as you can. You are on a strict time limit of fifteen minutes to reach the Sigil Stone. Proceed up the tower to the middle floor and then exit the door to a bridge. Go across to the next tower and then go around to another door. Exit out of this tower and run across another long bridge. Once inside the next tower, make your way down and exit the door there, to come to a large, long bridge that stretches out over the lava below. Near the center of the bridge, it will be broken, so hop across the gap (sheath your sword if need be to get some extra speed) and at the other side, look to the path that will lead to another World Breaker Guard Tower. Enter it and make your way to the very top to find a switch to open the War Gate blocking the main tower.
Once you hit it, run back down to the ground floor, exit the tower and go through the now open War Gate. In the center of the two gates, you will see the entrance to the large World Breaker tower. Now that you are inside, there will be a Dremora here who has the Sigil Key. Kill him, and then enter the door to the Vault of End Times and follow the path upwards –minding the blades that will come crashing down--and go to the door on the left side at the far end. Once through there, continue upwards and you will finally reach the door leading to the Sigillum Sanguis. Enter it with the Sigil Key and just like every other Oblivion Gate, go to the Sigil Stone and activate it. Once the flashiness of touching it is over, you will be back outside with Martin and the soldiers. Pick up the red and round Great Sigil Stone off the ground and talk to Martin to hand it over to him.
Once you arrive in Paradise, follow the stone path forward. You will run into some pretty mean enemies here, so take your time with them and defeat them as they come as to not have a group of them on you at once. Continue to follow the path and when it splits, follow it to the right. You will then come to a Dremora named Kathutet. He will praise you and tell you that you need the Bands of the Chosen to be able to reach Mankar. He will then offer you two choices; battle him or perform a task for him. Since he is rather easy to beat by hoping on top of the rock to the north and just keep pelting him with arrows. When he is down, you can take the Bands from him as well as his kick-ass sword and other loots he has. If you do choose to do the favor, you will need to travel to a cave and release Anaxes and then receive the Bands once the quest is complete.
Once you have the Bands, go to the east and go through the door there to enter the Flooded Grotto. Once inside, follow the water to the right and continue to go towards the quest marker. You will soon come to a red door that you can only enter with the Bands of the Chosen equipped. Put them on and then go through the door to the Forbidden Grotto. Once inside, go forward a little bit and you will face a Scamp and a Mythic Dawn agent. Kill them and then cross the bridge on the left. Once across, talk to Eldamil and tell him that you are there to kill Mankar. He will then offer to help you to remove the Bands, but needs you to act like a prisoner first for when Kynreeve shows up. Follow him to the cage and then step in when told to. It will lower and then rise, opening at the opposite side. Step out and Eldamil will tell you to meet him further along the cave. As you exit the cage, go through the cave opening and make a right.
Follow it to the next large room and then cross the bridge and go through the door on the right side. When you enter here, you will find Eldamil and he will offer to help you. Accept it and then continue forward. When you get into battle and if Eldamil falls, just wait a few moments and he will rise again as he is immortal. Once you have him as a follower, go south, down the corridor, and then east. Cross the lava by the bridge and continue northwest and then north. Continue to follow the caverns around and eventually you will come to Medrike, a mean Dremora with a big sword.
Have Eldamil help you take him down if you need a hand and be sure to loot his body for his Flame Sword afterwards. Exit out of the grotto and continue along the path in Paradise. Stick to the right and you will come to a set of stairs with Ruma and Raven on them, the children of Mankar, who you killed much earlier in the game. Raven was inside the sewers for the meeting and Ruma when she told you to sacrifice the person in the Dragon Shrine. They will talk for a bit and then lead you inside to face Mankar.
Once you are inside, Mankar will chat for a long while. If you wait for him to finish talking, all three of them will attack you at once, making the battle very difficult. Instead, while Mankar is talking, kill each of the offspring on either side, one at a time, since they will only attack when individually attacked before Mankar’s speech is done. When they are both down, interrupt Mankar’s speech by attacking him. Try to kill him as quickly as you can as Ruma and Raven will appear again shortly. It can be a tough battle but use your best weapons and don’t let up on Mankar. Try to get him cornered and keep on him till he falls. Once he does, loot all the bodies and get the Amulet of Kings from him. Once you do that, Paradise will start to collapse and you will be transported back to Cloud Ruler Temple.
Once you have returned to Cloud Ruler Temple, talk to Martin and give him the Amulet of Kings. He will tell you that now the two of you must travel to the Imperial City to light the Dragon Fires in the Temple of the One. Fast travel there and then make your way inside the Elder Council Chambers at the Imperial Palace to talk to Chancellor Ocato. He will then concede that Martin is the new Emperor, but a soldier will run in to interrupt and declare that the city is being overrun with Dremoras. Run outside with Martin and Ocato in tow and you will now need to defeat the onslaught of enemies while keeping Martin alive. Martin does a pretty good job of keeping himself alive and you needn’t worry about him much.
Once the first area is clear, go through the door to the
Temple District and head to the right. When you go far enough, you will see the
giant, red form of Dagon in the street. Go back and talk to Martin and he will
tell you that the both of you need to get to the Temple of the One to defeat
Dagon. Go along the road again to Dagon’s position and you can try to run past
him, but instead, go the much safer route, down the alley to the right and
follow it around the buildings and go behind Dagon that way. Once you come back
out to the main street, simply enter the temple on the other side of the road
and a cutscene will play, showing you the ending of the main quest!
Congratulations, Champion of Cyrodiil!