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