Doom Wiki

The Keymaster is a character in Strife. He is inside a small room next to the entrance to the Commons. From conversation, the Keymaster is a very minor role (basically, guarding a key), but he likes to think it is really an important role. After getting the key, he will just repeat 'I am the keymaster' trying to make it sound important.

SPOILER WARNING: Plot details follow.
Strife Keymaster Conv

You cannot enter his room until you have met the Oracle, which will say "take this token to the Keymaster", referring to the Oracle Pass. Once back at the Borderlands, ask the Keymaster to give you the proper key so that you can make your way to the Bishop's Fortress. He becomes irrelevant after that.

Spoilers end here.