When you're back to Cloud Ruler Temple, Martin will be ready to open a portal to Camoran's secret dimension. Get yourself ready (potions, items, u know) and let Septim open the thing. Go through.
Paradise is a not-so-peaceful land inhabited by Camoran's former servants, whose immortality proves to be the greatest curse of all. Follow the path and you'll meet one of them. Ask him about everything and continue on, across the bridge. Go straight ahead, twice turning right at crossroads.
You'll meet a dremora named Kathutet, who's guarding the exit from the gardens. He'll give you a choice: defeat him in a fight or perform a quest for him. If you choose the latter, a map marker for Lair of Anaxes will be added to your map. Go there and free Anaxes himself (by moving the two logs that keep the boulder in place), who's trapped in the back part of the cave. When you do that, go back to Kathuter (he's inside The Flooded Grotto). You can kill him now if you like. Either way, wear Bands of the Chosen and enter The Forbidden Grotto.
Talk to Eldamil, Camoran's former lieutenant, who wants to help you. Agree to that and step into the cage when he tells you to. When you're free again, go right, cross another bridge and use the door leading to the next level.
Eldamil will take off your Bands of the Chosen, enabling you to leave the grotto. Agree when he proposes to help you defeat Camoran. At the end of this quite long level you'll encounter a dremora named Medrike, who will have to be killed.
HINT: On the first level of the grotto you can kill the prison warder Orthe and his dremora sidekicks. They are quite a challenge, though.
HINT: If Eldamil dies, don't worry. He's immortal and will soon reappear.
Outdoors at last. Camoran's palace stand before you. You'll meet your old buddies Ruma and Raven near the entrance. They will lead you in. Mankar will give a standard villain speech and engage you into a fight. Don't bother fighting Ruma and Raven, as they're immortal as well and will respawn in a few seconds after getting killed. Concentrate on Camoran himself. When he dies, pick up the Amulet of Kings from his body. The Paradise vanishes, and you're back to Cloud Ruler Temple. Give Martin his legacy back.