Upon the opening of the three lesser gates, you will obtain this quest automatically once the Great Oblivion gate has been opened.
Dagon has opened the three lesser gates after a bold plan by Martin to allow a Great Gate, similar to the one that destroyed Kvatch, to be opened. It is now time to enter the gate and obtain the Great Sigil Stone.
When you arrive at Cloud Ruler Temple, Martin has bad news for you. He has deciphered the last item needed for the ritual to open a portal to Camoran's Paradise: a Great Sigil Stone. This has serious problems, as Great Sigil Stones are the tethers for Great Gates, the kind the Mythic Dawn opened to destroy Kvatch!
Jauffre doesn't like the plan, and this is why: Martin suggests that Countess Carvain allows the Mythic Dawn to proceed with their plan to open the Great Gate to attack Bruma. As such, Martin asks you to be his ambassador, to invite Narina Carvain to meet him at the chapel of Talos in the city. She agrees, and soon you witness the new Emperor's first diplomatic duty. Ask the Countess to stop having gates closed, and Martin will march out to meet his troops, to the applause of the citizens of Bruma.
Once you reach the battlefield, you will find one gate is already open. The battlefield isn't hard to find, just follow the long line of city guard. Martin will give a rallying speech, and then the Daedra will attack in earnest. Three daedra will spawn from each gate during each wave, and after every second wave another gate is opened. In total, you fight 36 daedra, with only limited time in between to heal and repair armour/re-apply spells.
Hopefully, during the quest Aid for Bruma, you have been collecting soldiers to help in the coming battle. If not, you're in trouble. To defend the Empire, there is a small contingent of Bruma Guards, including Burd, accompanied by Jauffre, Baurus (if he is still alive), Martin and yourself. Each city you have convinced to send guards will send a few guards, in some cases there will even be guard captains assisting in the fight. It is highly recommended that you bring along additional followers, as you will be facing a LOT of enemies, which depending on your level can range from scamps and Dremora churls to storm atronachs and Valkynazs.
Keeping Martin alive is ESSENTIAL. If he dies, the main questline is failed, nomatter what. Unfortunately, he is no longer marked as quest essential, so he can die. Do whatever you can to heal his injuries, and if you can, layer on protective spells! Mages find this easier, as custom spells can provide shield, reflect damage, resist normal weapons and other effects, all improving Martin's chances of surviving each wave. The Crusader's Relics from the Knights of the Nine expansion is a godsend (quite literally in fact!), as it grants you both a potent heal spell (Merciful Touch) and a quite handy shield spell (Blessing of the Eight), both of which only cost 25 magicka to cast. Even a warrior with no magical potency at all can still put out 2-3 spells each wave, frequently more than enough to heal and protect Martin. If neither of these are available to you, power attacks while the daedra are standing still (just after they leave the gate) can do enough damage to either kill or severely weaken the daedra before a guard (or Martin) arrives.
After the third gate opens, you have to battle two more waves before the Great Gate opens. As soon as you 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 you enter, so get inside as soon as possible. There is a 15 minute timer on this quest, however there is sometimes a glitch that makes it appear that you have two minutes less than you actually have: you can tell if this glitch affects you because the timer will start at 13 minutes instead of 15.
Immediately head towards the left and head for the first door to the Caverns. Inside the caverns, slay only the Daedra in your way and head for the door to one of the lesser towers. Inside this tower, head for the top. Kill the Daedra guarding the Corpse Masher, then ride your way up, 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 you can't finish the fight quickly either ignore them or knock them off the ledge into the pits of lava.
Alternatively, with a high acrobatics skill or a Water Walking enchantment, you can skip the caverns entirely and jump along the bottom of the broken bridge, carefully avoiding the lava. This can save you precious minutes at the cost of a few health potions. Another entertaining method is if you 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 you. It is rather difficult to get up to the door from the lava, though, so this is to be done purely for entertainment purposes.
Enter the tower. If you have more than 8-9 minutes, you can take the tower at a 'usual' oblivion gate speed, however do not be tempted to check every single loot container or body. Otherwise, you 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 you have lockpicks, ignore him. He has the Sigil key that unlocks Sigillum Sanguis, which is otherwise protected by an Average lock. Proceed up into the Rending Halls, killing any Daedra in your way. Be sure to use your map to avoid getting lost (there is a map marker). Exit back into the tower at the top of the halls, and continue your way up the tower.
If you are short on time, then ignore the Daedra or knock them off the ledge (if possible). The only Daedra you really need to kill is the Sigil Keeper since he has the key, and even he can be skipped if you have lockpicks, an Open Average Lock spell, or the Skeleton Key; the door to the Sigilium Sanguis is only protected by a pickable Average (three-tumbler) lock. Be sure to have brought lots of healing potions to this mission since there is going to be quite a bit of running (you don't have time to do much stealth). Also, enchanted weapons and armor help a lot.
Alternatively, if you're high-level alchemist or Illusionist, brew a few dozen Invisibility or Chameleon potions or bring an invisibility spell, like Deathly Visage, bring along lockpicks, an Open Average Lock spell, or the Skeleton Key, and simply run straight to the Sigil Stone.
After completion a statue will be erected in your honour by the citizens of Bruma near the northern gate. For this reason, you may want to bring along your favourite set of armor/weapons, for purely cosmetic reasons, as the most equipment you are wearing when you take the sigil stone will be what is sculpted. The statue is rather impressive, and certainly makes all that daedra- and undead-killing worthwhile.
While battling the waves of enemies waiting for the Great Gate to appear, if you acidently slay any of your allies during the battle, you may be fined, and upon arrival to Bruma after handing over the Great Sigil Stone to Martin, a Bruma Guard will confront you as a criminal. If you have any stolen items in your inventory, and you're unsure if you accidently killed any of your allies during the battle, you should save before going back to any of the cities so that you're not caught off-guard.
If you have certain enchanted items from the Shivering Isles, some weapons may not appear in your statue. When beat with Dawnfang/Duskfang, the character was depicted with a dagger instead.
A good way to cover a lot of ground quickly is to invest in 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.
If any of your armor/weapons are enchanted in some way (either by Sigil Stones or naturally magical) then the statue may wear unrelated armor/weapons. Ex: A daedric longsword with a lighting sigil enchantment became a staff and the Boots of Bloody Bounding became daedric boots.
Also, while the statue is usually grey, if the armour of the Divine Crusader is worn then the character is depicted in full colour.