Oblivion:Defense of Bruma

Warning!
Be sure to save before this part as there will be a lot of action going on at once and a game crash is common. Do not attempt to save in the middle of a wave of Daedra, as your game may freeze while saving, rendering it as an un-loadable corrupt file and forcing you to load from a previous save [citation needed].

Quick Walkthrough
# Ask Martin about the Great Sigil Stone.
# Escort Martin to the Chapel of Talos.
# Speak with the Countess in Castle Bruma.
# Make your way to the battlefield and eliminate all advancing Daedra.
# Keep Martin alive until the Great Gate opens, then enter.

Detailed Walkthrough
The Last Requirement
Just before you deliver the Great Welkynd Stone from the last quest, you'll see Martin (now hopefully wearing armor: if he is still in robes, see Bugs) and Jauffre debating about something. After they finish, talk to Martin, give him the stone and he'll fill you in on the last requirement: a Great Sigil Stone. The only way to get one is to allow the Mythic Dawn to open a Great Gate near Bruma.

Escorts through Bruma
Escort Martin, Jauffre and Baurus (if he didn't die in the Imperial City sewers during or after The Path of Dawn) to the Chapel of Talos, then go to Castle Bruma to speak with the Countess. Tell her the plan and she will follow you to the chapel where she will meet with Martin. This is the last chance you have to acquire Allies for Bruma.
If you neglected to obtain assistance from any of the other cities of Cyrodiil, your army will be composed of Burd, Martin, Jauffre, Baurus and some rather meager Bruma Militia.
If you have not yet completed Corruption and Conscience, Ulrich Leland will be sent from Cheydinhal to help defend Bruma.
If you have not yet started the quest, he will be flagged as an essential and hence will be unkillable in the Defense of Bruma - he can only be rendered unconscious. After the battle is done, you can return to Cheydinhal to complete Corruption and Conscience. If Ulrich was dead already, Cheydinhal contributes only one generic soldier to the cause.
If you have started the quest, Ulrich is no longer essential. As with any other action that leads to Ulrich's death, if he is killed in battle by any means, you will gain one Infamy point.
Speak to the countess when you are ready to begin the quest proper and she will order Captain Burd to organize the troops. Follow Martin past the cheering crowd and out of the city.

Lead the Charge Once More
thumb|right|Martin rallying the defenders of Bruma
Your army is waiting here and consists of Captain Burd and his guards plus any allies you gained in Allies for Bruma. Obviously the more troops you have, the easier the next part will be. Make your way to the battlefield where an Oblivion Gate is already open. Martin will rally the troops and a wave of Daedra will come through the gate. Depending on your current battle skills, you should be able to deal with the Daedra of the first gate by yourself. The troops won't attack until the Daedra come quite close to their line. Soon a second gate will open and waves of Daedra will come out of that too. Then a third gate and finally the huge Great Gate itself. Your objective is to keep Martin alive until you enter the Great Gate - if Martin dies, the game is immediately over and you must load a previous save. You do not have to kill every enemy in this mission (they will keep on spawning from the Oblivion Gates at regular intervals). Wait for the Great Oblivion Gate to open and enter the gate to complete this quest.
 

Strategy
You need to keep Martin alive — this makes the first part of the quest unique, and if you are not prepared, it could be a real headache. Also, note that you cannot leave the area once the battle has started, other than to enter the Great Gate. If you travel anywhere else, Martin will automatically die 15 seconds later. There are many ways to improve your chances in this battle:
Maximize the number of troops by:
Getting troops from all cities in the Allies for Bruma quest.
Getting as many Followers as possible to accompany you.
Preparing custom spells specifically for this battle:
Offensive spells with a large area of effect can be used to attack an entire group of Daedra when they first emerge from a gate, but you should also have a few spells with a small or with no area of effect to minimize the risk of hurting your allies. For example, a Command Creature spell can be used to cause the Daedra to fight among themselves, while not affecting your allies. A Paralysis spell can be used to disable your enemies to give you and your allies a better chance of defeating them. In general, try to avoid using too many damage spells (frost/fire/shock/wound, etc) with a large area of effect. Many of these will hit your allies, and may even hit Martin.
Defensive spells with a large area of effect and long duration can be cast on all of your allies before the battle starts. Some useful effects, even if they are small in magnitude (5-10 points) are: Fortify Health; Shield; Fortify Strength; Fortify Blade etc.
So that you can repeatedly cast these spells, bring plenty of Restore Magicka and Fortify Magicka potions.
Obtain zero-weight items (e.g., permanent bound items) and reverse pickpocket them onto your allies (especially Martin). The items can be enchanted for maximum effectiveness.
Use the Staff of Worms. This item is extremely useful during this quest because it can reanimate most, if not all NPCs for 30 seconds. A good strategy is to run around reanimating dead allies and fallen Dremora, having them fight for you.
Use Mehrunes Razor, obtainable after completing the Unearthing Mehrunes Razor official download quest. This weapon is extremely useful, since it has a chance to kill an enemy with a single blow (Daedra Banishing enchantment), no matter how tough it is. This is also extremely useful if you have a high Luck attribute, since the chance of an instant kill is boosted by your Luck points.
Remember, it does not matter how many Daedra you kill or how many allies die, only that Martin stays alive before the Great Gate opens. This includes Baurus and Jauffre, who will fight alongside all the guards. They may get killed, but it will not impact the storyline. You may create a custom spell to restore their health on Touch (with Target, they may move out of the way) and keep some Restore Health potions for yourself. Hotkey both of them, and heal yourself when your health gets low. After that, it simply becomes a matter of waiting.
If you have the Knights of the Nine official download and don't want to create too many custom spells, acquire the Crusader's Relics. This armor is extremely useful during this quest because of its lesser powers. For instance, use the Gauntlets' power to heal Martin, and the Greaves' power to protect him. Also having the full set bestows the Holy Aura, which provides a small, 5 pt Shield effect.
Watch who you hit in this battle! As is usual, you can hit your own allies when you fight or cast spells, but if you accidentally hit your allies and land the killing blow, you gain a bounty of 1000 gold and a point of infamy. It is possible to be simultaneously hailed as the hero of the battle and be arrested for murder walking back to Bruma.
Play carefully. Archery will allow you to hit Daedra that have just entered the battlefield, but once they start brawling with the soldiers, they move too fast and the chance of hitting one of your allies is too great. Similarly, charging in and blindly swinging a sword will do more harm than good. However, having a fast weapon (usually a one-handed weapon) pays off here, especially considering the number of enemies.

Once you enter the Great Gate
As soon as you enter the Great Oblivion Gate, the game causes everything attacking Martin to stop, so don't worry about him being killed. As soon as you retrieve the Great Sigil Stone, Martin will once again be essential. Other NPCs may still die, however.
When you enter the Great Oblivion Gate, if your speed is low, you may wish to ignore the Daedra and make a beeline for the Sigil Stone. If you keep running, you should be able to make it before even half of your time limit expires.

Notes
You may want to do this quest in low travel times, for several people who travel can walk by the battle and get involved in it, and most likely die. Surprisingly, there are many people who may stumble upon the historic moment. They are: Algot the Northerner, Alval Uvani, Dro'Nahrahe, Kellen, Romana Faleria, Salomon Geonette, and Stantus Varrid.

Bugs
 
 If Martin is in his robes when you march into battle, open up the console, click Martin and enter:
::: 
::Then click on him again (so that Martin's ID clears from the top of the screen), and type  
:*It is also possible to reverse pickpocket permanent Bound Armor and Weapons into Martin's inventory. This is the most appropriate 'fix' if you do not wish to reload a previous save.
 
Use the code   to make the Great Gate appear and go through it. It will skip the Battle of Bruma quest but when you come out of the gate Martin will be there.

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