Quest: Blood of the Divines

The undead Blades should all have nice magical equipment.

After obtaining the Daedric Artifact for Martin (or after enough time has passed since he gave you that quest), he'll tell you of another item he'll need to open the gate: the blood of a divine. Gods don't normally shed blood, but at least one of them was once mortal: Tiber Septim, the founder of the Empire. His armor is enshrined in the catacombs of Sancre Tor, as Jauffre will tell you when you ask. You'll have to go there and find the armor, even though no one has ever visited the catacombs and come out alive....

Get to Sancre Tor. It's probably easier to head north from Chorrol than it is to leave from the Cloud Ruler Temple, but either way you're going to have to get pretty high up on a hill, so bring a horse along. (If you find yourself crossing a lot of wooden bridges way up in the mountains, odds are you're heading towards Hermaeus Mora's Daedric shrine, which you're exceedingly unlikely to be able to use at this point.) When you reach the castle, take the stairs near the door to search the chests upstairs, then head inside. Be sure to bring along either a magical or silver weapon when you enter Sancre Tor, as there'll probably be ghosts inside, depending on your level.

(Maybe it was just us, but the dagger by the skeleton immediately after you enter the tomb was magical in our case, so be sure to check it out before moving on.) Fight your way through your enemies until you reach a Skeleton Blade, who wields Blades weapons and armor. Kill him to free his enslaved ghost, which will tell you of the vicious Underking, who rose and defiled the temple in the tomb. In order to cleanse the temple and pay homage to Tiber Septim, you'll have to free the spirits of the other Blades trapped in their skeletons. Search all of the treasure chests in the area before heading into the Entry Hall.

Free the Skeletal Blades

There are three more cursed Blades scattered around Sancre Tor, so track them down (they appear in red on your radar) and free them all by killing them. Be sure to check their equipment to see if it's magical before moving on, as much of it will be if you're beyond level five or so.

Jearl and Saveri aren't exactly subtle about their Mythic Dawn leanings.

When you've killed all of the Blades, head to the Catacombs, where their spirits will open the way to the tomb of Tiber Septim. There's nothing special you have to do here; just grab the armor and get the heck out of dodge. If you like, feel free to wear the armor yourself; it weighs nothing and looks pretty snazzy. Give the armor to Martin and he'll refer you back to Jauffre for your next quest.

Quest: Bruma Gate

As promised, an Oblivion Gate has opened outside of Bruma, and Jauffre wants you to help the Bruma guard shut it down. Not much to say here: you're going to have Burd along, as well as a couple of grunts, but the grunts are unlikely to make it through the experience alive, and Burd himself will probably also bite it eventually, although he can heal himself like many other important characters. Beyond that, there's nothing mysterious here, and nothing you haven't probably done already: just take the path around to the base of the tower, enter it, head to the top, and grab the Sigil Stone before getting out of town. Nothing to it!

Afterwards, return to Jauffre and give him the update. He'll ask you to find some Aid for Bruma.