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