In order to destroy the Master, the player first needs to reach the Cathedral, located south of the Boneyard. Accessing the Vault in order to confront the Master requires first getting into the basement.
Diplomats can wear robes or carry the red COC badge and as long as they have no party members or the invaders on their tail, the nightkin won't attack. They can talk to Morpheus, reveal their identity as the Vault Dweller, then agree to meet the Master. This instantly transports them to the Master's chambers for a diplomatic confrontation (see below).
Stealthy characters can use the robes and try to assassinate Morpheus to loot the badge from his corpse. With 100% Lockpick or better, it's also possible to open the door into the basement.
Reaching the Master
To open the secret door to the lower right, the player can use Traps or Lockpick on the lowermost bookcase to reveal a secret lever, then use it as normal. Otherwise, they can either blow up the door or wait for Jeremiah to open it. Jeremiah will ignore the player if they are wearing robes, otherwise, he will either throw them out of the Cathedral or have to be killed.
From this point on, the combat route is to just kill everything in the way to reach the elevator to level 3 and kill the Master.
Diplomats and sneaks cannot have any part members or invaders, carry a weapon, and must keep on wearing the robes. To enter the Vault, the player has to bluff their way in (worth 750 xp and requires not asking about things that would be obvious to Children with the right security clearance).
The computer just beyond the airlock can be hacked with Science to learn the location of Mariposa Military Base. This grants 1250 xp even if the location was previously visited. The elevator connects to the rest of the Vault.
Level 2 should be visited to gain the psychic nullifier from the psykers locked up near the chapel. Gideon will give one if the player has 8 or more Intelligence. This is worth 1000 xp. Otherwise, he has to be killed or the nullifier stolen.
Level 3 has the mutant barracks, command center, and Master. Ignore Jeremiah, as he will always attack. Viscious will attack if the player has weapons drawn, otherwise he can be ignored. The other elevator leads to the nuclear bomb and the stealthy method of defeating the Master.
The Corridor of Revulsion will deal damage to players attempting to reach the Master (unless the Nullifier is equipped or Mental Block was selected during a level-up). There will be six attacks in total:
Entrance (no effect): "The hairs on the back of your neck rise. You feel something from the corridor ahead. The bile rises as your recognize the mass of flesh ahead - human parts in the wall itself. The worst part is that it is moving, calling to you."
"Your nose starts to bleed. You hear voices in your head. They are calling your name. Darkness washes over your eyes for a moment, but you recover." (direct damage)
"A voice calls to you. You turn you head and see the image of a forgotten friend, obviously dead. The worms fall from her rotted jaw as your headache grows." (direct damage)
"It is getting harder to move forward. Your foot seems bonded to the floor. You look down, and the flesh of the floor is crawling up your leg. Teeth gnaw at your knees. Still, your headache gets worse." (direct damage)
"Something catches your attention, a flash of movement from the ceiling, and then a sliver of bone pierces your eye! With it comes the thoughts of the dead and tortured!" (direct damage, causes eye damage, and permanent -1 to Perception)
"The pain in your head is maddening. You feel the sins of others wash over your soul. And it gets worse, as your sins answer them. Like madmen, they fight through your psyche, and in the process tear your personality apart." (-1 Intelligence permanently; deals major damage to Childkillers and Berserkers)
"Your head is pulsing with the intrusive thoughts of others. The pain grows and grows. Suddenly, as if it never existed, the pain vanishes. You fall to the ground and weep. But something does not feel right." (-1 to Intelligence and Perception permanently.
As a result, the Corridor will result in major damage, crippled eyes, and -2 to Intelligence and Perception. Followers (except for Dogmeat) are affected as well, unless the player has the Psychic Nullifier. Mental Block does nothing.
Defeating the Master
Combat includes facing the Master directly. He has 500 hit points, a Gatling laser, and thick armor. Furthermore, he will spawn a super mutant in the corridor every two turns (four at the lowest combat difficulty), up to a maximum of twelve. Hardened power armor and high damaging weapons are a necessity to withstand the damage output, however, the Master will not attack companions or his Floating eyes. In fact, the robots are not aggressive towards the player and if splash damage hits them, they will turn on the Master. He is also vulnerable to pulse grenades. Saving and reloading will also cause the spawned mutants to disappear, but still count towards the maximum of twelve.
Diplomacy requires the player to have read Vree's autopsy report and have at least 7 Intelligence and good Speech. Then it's a matter of asking him about the plan, pointing out a problem with it and either giving the disk or telling him to ask his female mutants.
Stealth involves no confrontation and infiltrating the lowest level via the barracks in the command center. The doors are locked and require a good Lockpick. The player can activate the bomb after killing the guards (they don't take kindly to tampering) by using Science (70% required), Lockpick (70% required; the test was intended to be for Repair, but the game checks for Lockpick) or the nuke key looted from Mariposa.
Finally, it is possible to join his cause simply by talking to him and agreeing to his demands or persuasion. Doing so triggers the dipping ending and the raid on Vault 13, but grants no end slides.
As a result, the nuclear weapon is activated. The player has 240 seconds to reach the exit grid outside the Cathedral.
10000 XP for killing the Master (plus combat experience).