Go to the Cathedral, and kill everyone there. Loot a CoC Badge, and open the door near the stairs that go up.
There is a nightkin. Kill it.
Go downstairs and you end up in a cave/room of some sort.
http://img199.imageshack.us/img199/93/fo1cathedralgroundfloor.jpg
When there, move to the end of the room where touching a bookcase (or lockpicking, can't remember) opens a secret door.
Kill everyone as you proceed into the base. Wear CoC robes if you don't want to fight since you will not be hostile then to anyone. If someone asks, try to talk your way out of the situation.
You end up in a vault. First two levels include basic kill n' loot action, but in the east end of Level 3 you find a cave which will lead to the Master.
You can kill him by either talking him out of his plans, detonate a nuke or basically shooting him till he dies. A regular late-game weapon should do. Aim for the eyes.
Slasher (the guy I point to kill in the pic) may not be the one holding the necessary badge. If it doesn't work, then try the other high-ranking CoC members in the building. Morpheus, upstairs, is my second choice.