Great Gate

Mehrunes Dagon has opened the three lesser gates after a bold plan by Martin to allow a Great Gate, the largest and most deadly gate, similar to the one that destroyed Kvatch, to be opened.

After fighting three attacks of Daedra with the help of the men from all cities it is now time to enter the gate and obtain the Great Sigil Stone. One will obtain this quest automatically once the Great Oblivion gate has been opened.

Three gates opened
After the third gate opens, the Hero have to battle two more waves before the Great Gate opens. As soon as one see the gate begin to rise up out of the ground, get into position to run into the gate. The battle raging outside pauses when they enter, so get inside as soon as possible.

Inside the Great Gate
After entering, immediately head towards the left and head for the first door to the Caverns. Inside the caverns, slay only the Daedra in their way and head for the door to one of the lesser towers. Inside this tower, head for the top and kill the Daedra guarding the Corpse Masher, then ride up to the top, killing any other Daedra in the way.

At the top, activate the Gate Control, then head back down through the door. Go past the opening gate, and deal with the Dremora. If one can't finish the fight quickly either ignore them or knock them off the ledge into the pits of lava.

Alternate ways

 * Alternatively, with a high acrobatics skill or a Water Walking enchantment, one can skip the caverns entirely and jump along the bottom of the broken bridge, carefully avoiding the lava. This can save them precious minutes at the cost of a few health potions.
 * Another method is if one have mastery in Acrobatics: Water-Jump the entire way across the lava! A solid absorb magicka, reflect magicka, or resist fire enchantment will be useful to avoid all of the fireballs the Siege Engine is throwing at them. It is rather difficult to get up to the door from the lava, though, so this is to be done purely for entertainment purposes.

The Tower
Enter the tower and if there is more than 8–9 minutes, one can take the tower at a 'usual' oblivion gate speed, however do not be tempted to check every single loot container or body. Proceed up into the Rending Halls, killing any Daedra in their way. Be sure to use the map to avoid getting lost (there is a map marker). Exit back into the tower at the top of the halls, and continue up the tower.

One can run through and basically ignore anything that takes more than a few swings or spells to kill. Either kill the Sigil Keeper, or if they have lockpicks, ignore him. He has the Sigil key that unlocks Sigillum Sanguis, which is otherwise protected by an Average lock.

Alternative way
If they are really short on time, then ignore the Daedra or knock them off the ledge (if possible). The only Daedra they might need to kill is the Sigil Keeper since he has the key, and even he can be skipped if they have lockpicks, an Open Average Lock spell, or the Skeleton Key.

When inside the Sigillum Sanguis, head up to the top to remove the Sigil Stone. The Great Gate will shut down and the Hero is back on the battlefield where all the soldiers are clapping their hands of their success.

Great Gate closed
Find Martin and give him the Sigil Stone, he will be very pleased at the success of closing the gate and will proceed to Cloud Ruler Temple to prepare for the next step.

Quest is completed.

Tribute to the Hero
After completion of the quest a statue will be erected in the Hero's honour by the citizens of Bruma near the northern gate.

The statue is rather impressive, and certainly makes all that daedra- and undead-killing worthwhile. This Hero is wearing Umbra's Ebony Armor, with Helm of Oreyn Bearclaw and Hatred's Soul.

Trivia

 * While battling the waves of enemies waiting for the Great Gate to appear, if one accidentally slay any of their allies during the battle, they may be fined, and upon arrival to Bruma after handing over the Great Sigil Stone to Martin, a Bruma Guard will confront them as a criminal. If there are any stolen items in the inventory, save before going back to any of the cities so that one is not caught off-guard.
 * There is a 15 minute timer on this quest, however there is sometimes a glitch that makes it appear that one have two minutes less than one actually have: one can tell if this glitch affects you because the timer will start at 13 minutes instead of 15.
 * If there are certain enchanted items from the Shivering Isles, or any other enchanted armor and weapons, some may not appear in their statue. When beat with Dawnfang/Duskfang, the character was depicted with a dagger instead.
 * Also, this is the only quest where Jauffre can (and most likely will) die. Baurus may also be vulnerable once again.
 * If they are a high-level alchemist or Illusionist, brew a few dozen Invisibility or Chameleon potions or bring an invisibility spell, such as Deathly Visage, (available to members of The Dark Brotherhood from the Khajiit M'raaj-Dar). Using an invisibility spell will mean that you can bypass any fights with Daedra or Dremora that you encounter in the tower and cave system. Having recently completed the Miscarcand quest, you might have a large quantity of Welkynd Stones in your inventory, handy for refilling your magicka, as you will have to cast the invisibility enchantment regularly as it dispels whenever you walk through a door, activate anything or attack.
 * It is possible, when activating the Great Sigil Stone to grab it, to get multiple of them by activating repeatedly.This is unnecessary however, since they can not be used.
 * Also, while the statue is usually grey, if the armour of the Divine Crusader is worn then the character is depicted in full colour. For this reason, one may want to bring along your favourite set of armor/weapons, for purely cosmetic reasons, as the most equipment they are wearing when they take the sigil stone will be what is sculpted.