Here is the full list of all 12 Tetragon achievements.
Find and interact with all ghosts of your son
Finish game with all the fragment of Tetragen
Find and collect all the fragments of the Tetragen
Find the secret level of Deep Woods
Find the secret level of Fruitful Forest
Entry on the portal on the Dungeon Level
Complete all levels of Ancient Stones
Finish game without all the fragment of Tetragen
Complete all levels of Deep Woods
Complete all levels of Gnik Castle
Release the creature on the last level of Gnik Castle
Complete all levels of Fruitful Forest