When you're back to Cloud Ruler Temple, both Martin and Jauffre will have some stuff for you to do. The former translated another part of the book. Talk to Jauffre, who'll tell you to go to Sancre Tor in order to find Tiber Septim's ancient armor. He'll also give you a key enable you to enter the dungeon.
On the first level you'll be attacked by an undead Blade. When you defeat him, his spirit will appear and tell you to free the other three Blades just like you did with him. Go to the lower level. Now you're in the center of the dungeon. Firstly, go to Sancre Tor Prison.
Kill Warder Kastav and get a key from him. Find the undead Blade (follow your compass) and defeat him. You'll find some nice equipment on him. Go back to Entry Hall and continue to Hall of Judgement, where you'll have to kill another Blade. The last one of them is in Catacombs. When you have them all "freed", go to Entry Hall once again and to Tomb of the Reman Emperors. All 4 ghosts will join their powers and lift the curse, allowing you to enter and grab the armor. Do so and go back to Martin.