Secret achievements are hidden landmarks found in certain maps during gameplay. To unlock a secret achievement, one just has to visit the hidden landmark!
Collect all 13 secret achievements to unlock the 'Explorer' and 'Seeker' Achievements!
Found in the bronze level of The Undergarden.
Heart of the Faceless
Found in the gold level of Sealed Tower.
Interestingly, this secret achievement is unlocked with the viewing of the apprentice's diary (by reading the journal entry found in the room).
Found in the bronze level of Homunculus Bunker (Master).
Similar to the 'Apprentice's Quarters' achievement, 'Mystery Mage' is unlocked right after reading the letter left behind by Harbinger in the secret area. This time, the secret area requires prior knowledge on how to unlock it, and the instructions can be found in a journal entry left behind in the gold level of The Crypt (Master). (You can find the screenshot of the instructions here )