Quake Easter Egg - Secret Credits Room

1. Work your way to Episode 2 Map 7, The Underearth.
2. Move through the level as normal.
3. You will see pictures of a demon on walls through-out the level.
4. Shoot all of them and you will see a small amount of blood.
5. Keep going until you get to a hallway where there are two gaps in the wall, one on the left and one further on to the right.
6. Go to the second one on the right and shoot the wall.
7. After the wall has opened you will be in a secret room.