Everything from Martin's end should now be ready. He has the Great Welkynd Stone, the Great Sigil Stone and the Mysterium Xarxes and can now open the portal in to Mankor Camoran's Paradise. He warns you that this is a one-way trip because the portal will close behind you, so take whatever time you need to get any equipment, spells or other resources you might need. You're on your own for a while, so make sure before you activate the portal. If you didn't notice at the time, the Great Welkynd Stone and Great Sigil Stone were consumed and disappeared when the portal opened.
At first glance, Paradise is exactly that. Beautiful landscape, plants, beautiful sunset...a seeming idyllic place. Well, at least until Camoran starts talking to you. He'll start preaching to you at a couple or three points. You can listen or not as the mood takes you. Getting through this mission involves navigating three areas: The Savage Garden (where you are now), the Flooded/Forbidden Grottos (an underground transition area) and Carac Agialor (where Camoran is).
Budding alchemists will love Paradise; ingredients all over the place. Combat-types will also love it; respawning Daedra. Everyone else will probably just want to get to Camoran as quickly as possible. Aside from the ingredients and Daedra, there is only one pile of loot on the overland map - an Ayleid Chest on an island on the far eastern side of the map. You'll probably need to swim along the coastline to get to it, though. The overland route may be beyond even Masters of Acrobatics. And the island is home to what may be the one and only Spectral Mud Crab in the game (the only one that I've ever run into, at any rate).
So whenever you get tired of collecting ingredients, killing daedra or whatever is keeping you occupied at the moment, head east to the center of the island. You'll be met by a Dremora named Kathutet. You defeated his kin at Kvatch and he has lost honor because of it. There are two ways that you can satisfy his honor and receive the Bands of the Chosen, which will let you into the Forbidden Grotto. You can fight him (and win) or you can serve him. If you're confident of your combat abilities, fight. He's five levels above you, but otherwise behaves like a fighter-type Dremora.
If you're not so confident of your combat abilities, offer to serve ("I have questions" gets you nowhere). The mission is simple and shouldn't involve fighting anything except whatever Daedra you might encounter on your way there. He wants you to free a Xivilai named Anaxes who managed to get himself trapped by some of the Ascended Immortals that he was supposed to be torturing. Anaxes is in a cave a little way to the northwest of where you encountered Kathutet. The three Ascended Immortals in the cave may not be aggressive at all (they haven't been when I've gone in, but your mileage may vary). Go to the back of the cave and activate the two logs holding the boulder in place. This will free Anaxes, who will then proceed to kill any Ascended Immortals he can find (including going out onto the overland map). Head back to the Flooded Grotto to meet up with Kathutet.
If you freed Anaxes, Kathutet is as good as his word and hands over the Bands of the Chosen. If you killed Anaxes, you'll have to fight him and get the Bands off of his body. Once you have the bands, go to the entrance of the Forbidden Grotto. You have to wear the bands to activate the door, but once you put them on, you can't take them off until someone removes them for you. The Bands give you a 50% weakness to Fire. If you're a Dark Elf, you're still 25% to the good; if you're a High Elf, this could hurt a bit, especially if there are Dremora and Flame Atronachs tossing spells at you.
When you enter the Forbidden Grotto, you'll understand the weakness to fire. The Forbidden Grotto isn't a way out of Paradise, it's just another prison where Camoran's Dremora torture the immortal prisoners by dipping them in lava. Shortly after you enter, you'll meet up with a High Elf named Eldamil (he's essentially a Mage at your level), who offers to help. You'll need at least a little of his help since he's the one who is going to open the door that lets you continue on (he'll probably get himself killed covering your exit, but that's OK - he'll respawn). Make your way through to the door to the next section of the Forbidden Grotto, where Eldamil will meet up with you again and remove the Bands of the Chosen. Interestingly enough, they were necessary to get into the Forbidden Grotto, but you can't get out of it if you're wearing them. He will offer to go with you and even though his appearance seems just a little too coincidental, he's really on the up-and-up. He'll probably manage to get himself killed before you ever get out of the Grotto.
Once you're out of the Grotto, you'll be at the foot of some stone stairs leading up a hill. You'll also see a lot of Mana Blooms along the way. Alchemists take note. You'll find a lot of Mana Blooms around here. The purple ones give you Ambrosia (Restore Health) and the blue ones give you Lichor (Restore Magicka). Wonderful stuff since the only have one effect and will combine with other ingredients to produce a useful potion that can have a long duration and no side effects. When you get up to the top, you'll run into Raven and Ruma Camoran again (why is it that some people just can't stay dead?). Don't bother killing them; they'll just respawn inside and you'll have to do it all over again.
Make sure that you have gathered whatever ingredients you want because you can't come back here and you can't stay to collect anything after Camoran is dead.
Enter the palace and you'll see Mankar Camoran sitting on his throne in front of you. Mankar is your target. He is 10 levels higher than you and has a robe with Reflect Spell and Reflect Damage, so be cautious or have some healing available if you're going to break out the heavy weapons or spells. Aside from Raven's Staff of Fireballs, those two don't have anything worth picking up, so you can let Eldamil run interference while you concentrate on Mankar (he's going to die when Mankar does, so don't be too concerned for his well-being). When he's dead, you'll have about 15 seconds to take whatever you can or the Amulet of Kings will be placed in your inventory and you'll be transported back to Cloud Ruler Temple. Just for the record, the Mysterium Xarxes disappears from the game when Mankar dies.
Once back at Cloud Ruler, you'll be hailed as the returning hero by whatever Blades are there and you can give the Amulet to Martin (who is now wearing the Emperor's robes). He sounds a little hesitant about putting on the Amulet, but does it and you can be the first to officially call him "Your Majesty". From here, you'll need to go to the Palace in the Imperial City so that Martin can meet with Chancelor Ocato, which kicks off the next (and last) phase of the Main Quest.