The last piece of the Misterium Xarxes puzzle is a Great Sigil Stone. The problem is that the only chance of getting it is by allowing the enemy to open a Great Gate. Martin's plan seems desperate, but that's the only choice you have. Bruma must survive the attack. Septim will tell you to inform Narina Carvain about everything. Co to Castle Bruma, then.
It turns out that the countess is quite willing to take the risk. She'll promise to meet Martin in Chapel of Talos. Now is the time to finish up with Allies for Bruma quest - if you're not afraid to loose, of course. When you think that you're ready, talk to the countess.
Everyone will be waiting by the city gates. Go there and watch the scene. The number of soldiers that is fighting on your side varies depending on how much allies you found earlier. After some speech by Martin, battle begins. Fight off enemies until a Great Gate appears. When it does, go through it.
A giant siege engine is approaching the gate from this side. You have to get to Great Sigil Stone before it reaches your world. You've got 15 minutes. Go inside World Breaker Guard through the door on the right. Get on the platform and hit the mechanism to go up. Go to the top and leave through the door. Cross the bridge to another tower, then go down to the middle level and go out. Jump across the broken bridge. There is a path on the right that'll lead you to the next tower. Follow it, avoiding the fire from turrets. If you don't manage to jump across, go left to enter a short dungeon. Either way you'll eventually end up in of the two identical towers. Go to the top and use the lever to open the gate. Go outside (through the middle-level door) and run to the main tower.
Go to Vaults of End Times. Defeat a Sigil Keeper and take his key. Go upwards until you reach Sigillum Sanguis. Touch the stone. It's over, you've won this battle. Bruma is safe. Talk to Martin. It's time to pay Camoran a visit.