Here is the full list of all 13 Depths of Fear :: Knossos achievements.
Complete 15 'unknown' levels in a row
Defeat the Minotaur
Destroy 100 monsters
Defeat the Hydra
Defeat the Medusa
Defeat the Griffin
Defeat the Manticore
Defeat the Centaur
Defeat the Cerberus
Complete 5 'unknown' levels in a row
Destroy 25 monsters
Defeat the Satyr
Level complete without being seen