Secret achievements are hidden landmarks found in certain maps during gameplay. To unlock a secret achievement, one just has to visit the hidden landmark!
There are a total of 13 hidden landmarks - the first 12 can be found in Story Mode, while the last one can only be found in Master Mode.
Collect all 13 secret achievements to unlock the 'Explorer' and 'Seeker' Achievements!
Jumbo Veggies
Found in the silver level of The Garden.
Sister Anvil
Found in the gold level of Ruin Crossing.
Warchief Tent
Found in the silver level of Orc Forest.
Slime Laboratory
Found in the silver level of Slime Cave.
Cave Painting
Found in the bronze level of The Boarlands.
Golem Forge
Found in the gold level of Tetramont's Estate.
Ruined Golems
Found in the bronze level of The Undergarden.
Vataneba's Portrait
Found in the silver level of The Crypt.
Demon Shrine
Found in the gold level of Ashen Cult.
Eye Tanks
Found in the silver level of Temple of Blorpx.
Heart of the Faceless
Found in the gold level of Homunculus Bunker.
Apprentice's Quarters
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).
Mystery Mage
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 )