Find a super secret

Complete Farewell to Arms on any difficulty

Kill yourself 10 times in single-player

Complete Post Mortem on any difficulty

Find the secret level in Post Mortem

Complete Even Death May Die on any difficulty

Complete Ghost Town (E3M1) on any difficulty without any Innocents getting killed

Kill a Choking Hand

Find the secret level in The Way of All Flesh

Complete The Haunting (E2M5) in under 40 seconds on any difficulty

Find the secret level in Cryptic Passage

Complete Dead Reckoning on any difficulty

Find the secret level in Even Death May Die

Kill 100 Cultists

Find the secret level in Farewell to Arms

Defeat the 3 Beasts of Beauty and the Beast (E6M8) in under 3 minutes on Well Done difficulty or above

Exploit the Hellhound's greatest weakness

Find the secret level in Dead Reckoning

Complete Cryptic Passage on any difficulty

Complete The Way of All Flesh on any difficulty