The Vault Dweller has trekked across the wasteland and brought the water chip back to Vault 13, but the mutant armies still pose a dangerous threat to the safety of the vault populace. It is up to the Vault Dweller to stop their mad leader, the Master, as well as the Forced Evolutionary Virus, from which they originate.
|Main Quest: Destroy the Mutant leader|
|Go to the Cathedral.|
|Speak with Laura, mention the password and ask her to open the tower door.||Acquire a red COC badge and unlock the tower door.|
|Go to the 3rd floor, kill Morpheus and take the black COC badge.|
|Go back and use the black badge to unlock the basement door.|
|Enter the basement and go through the secret door to the vault.|
|Go to the third level.|
|Free the prisoners (optional).|
|Sneak behind the mutants, enter the lift and activate the bomb.||Negotiate with the Master.||Kill the Master.|
|Get out before the bomb explodes.|
|Reward: 10000-12000 XP|
End of the game if completed Destroy the source of the Mutants
The seemingly peaceful Children of the Cathedral have, up until this point, been a helpful organization, with their free hospitals in Junktown, Necropolis and the Hub. Now, the Vault Dweller needs to enter the Cathedral's walls to discover and defeat the evil within.
The Cathedral is located two squares south of the Boneyard and with Katja being the only person able to mark it on the Vault Dweller's map, one may need to search to find it. Once there, go through a staircase in the back of the main room. However, the door leading to that room is locked. To get there, a red COC badge must be obtained. Most of the Children in the main room, like Lasher, will carry one.
Once the stairwell is reached, follow it down. However, another locked door blocks the path. Head up the stairs nearby and locate Morpheus on the third floor. The nightkin along the way up will be hostile to the Vault Dweller unless they are wearing robes, are unarmed and alone. Morpheus' death awards 1000 XP, and the black COC badge. Alternatively, he can be convinced to take the Vault Dweller directly to the Master (see Speech solution).
At the bottom of the hidden staircase, there is what appears to be a dead-end library. Searching the wall on the east side will reveal some intricate grooves. Lockpick or search the nearby bookcases for traps to reveal a lever and open a hidden passage. If it is after 8 P.M., there will be a centaur and two floaters out in the caves, along with two armed super mutants regardless of the time.
Enter the Los Angeles Vault. The first level is home to several more hostile super mutants the Vault Dweller will have to defeat to reach the elevator to get to the third floor.
Once there, the player character is attacked by several Mister Handies, floating eye bots, and more super mutants. These must be defeated before heading down the corridor to where the overseer's chair would be.
As the Vault Dweller walks down the corridor, they will get progressively worse messages in the message box, as well as taking Eye Damage, losing 2 points of Perception and Intelligence each by the time the far side is reached. At the very end lies the Master. If the player has the Mental Block perk or a psychic nullifier from one of the psykers on the second level, they are immune to the damage.
The Master is a very powerful foe, with twin (although dealing damage as one) Gatling lasers, support from two floating eye bots and a contingent of super mutants. Defeating him will result in 10,000 combat XP.
Once the Master is gone, the entire Cathedral begins a self-destruct sequence; at which point a timer will come into effect. If the Vault Dweller does not make it out in time, they will die. As when the Vault Dweller blew up the Mariposa Military Base, make sure to leave the map. Simply being outside the building won't count as having escaped.
If the Vault Dweller does not want to kill the Master in combat, he can also be talked out of his plans for the Unity. The Vault Dweller can convince him to stand down with the use of a very high Speech skill.
Alternatively, he can be confronted with a slightly lower Speech skill and Vree's autopsy report. The Vault Dweller must have an Intelligence skill above six to obtain the tape from Vree. Once they read the holodisk, they can show the Master that all mutants are sterile. He will be unable to cope with the things he did in the name of the Unity, which he now knows to all be in vain. He will then tell them to leave "while you still have hope" and to find safe ground. He will commit suicide by detonating a nuke beneath the Cathedral.
- It is important to note that the use of certain perks negates the Speech alternative and makes this ending unavailable, even with maximum Speech level and Vree's tape.
If the Vault Dweller has either the Lieutenant's key from Mariposa Military Base or a very high Science skill, that same nuke may also be used by the Vault Dweller. Get to the third level, but take a left once the intersection is reached. There will be a few more super mutants and an elevator behind them. Enter that elevator. Two mutants guard the nuke and once they're taken care of, use the key to set the nuke for a four-minute countdown. Be careful, if the Vault Dweller is still on the Cathedral map when it goes up, they go with it.
- 10000 XP for killing the Master (plus combat experience).
- 2000 XP for freeing prisoners.
- Ending, if the Vault Dweller first completed the Destroy the source of the Mutants quest.
- If the Vault Dweller convinced the Followers of the Apocalypse to help defeat the Master there will be four Follower invaders on the map waiting for the Dweller. They are each wearing metal armor and are equipped with melee weapons. One has a sledgehammer, one has a Ripper and two have spears. Also, unlike the Brotherhood's "crack assault paladins," these people will actually follow the Vault Dweller inside the Cathedral and the vault.
- One can also convince the Brotherhood of Steel Elders to send help.
- The super mutants guarding the bomb are much easier to kill than the other ones present throughout the Cathedral, able to be defeated with one or two blasts from heavy weapons.
- With a high companion count and the number of foes on the upper Cathedral floors, it is very likely that the Vault Dweller will not be able to move due to there being too many characters in one place.
- If the Vault Dweller has received the password from Nicole they will also be able to talk to Laura and tell her the password "Red Rider" to get through the red COC badge door.
- If the Vault Dweller takes any companions with them up the stairs, or is not wearing robes, the nightkin sentries turn hostile.
- Having Morpheus bring the Dweller to the Master will make it harder to escape.
- If the Vault Dweller has an Intelligence below 4, the Master will say he doubts even the FEV will help them, and wonder why he is talking, adding that it is unlikely they even understand.
- Even if Mariposa has previously been destroyed, the option to join the Master and get dipped is still present. The ending will still play out as usual.
- The speech given by Jacoren at the end of the quest is referenced in Fallout 3 by Amata after the Trouble on the Homefront quest.