Acquire all 3 Demon Key Artifacts in the main campaign
Finish the Lost Levels mission
Defeat the Mother of All Demons on The Absolution
Finish a 'Fun' Level
Locate the secret level 'Hectic'
Find the BFG9000
Find the Unmaker
Finish 'No Escape' on Watch Me Die! skill level
Discover a secret area
Kill a Cyberdemon