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