Here is the full list of all 19 Obscure achievements. It takes around 12-15 hours to unlock all of the achievements on Windows.
Finish the game in 'Hard' mode.
Finish the game in 'Special' mode.
Kill 150 monsters with a firearm.
Every playable character survived at Leafmore.
Finish the game in 'Normal' mode.
Finish the game.
Kill 3 monsters with a single light grenade.
Find all of the weapons.
Cross the gardens.
Enter the gardens.
Check out the dorms.
Search the basement.
Search the auditorium.
Go to the nurse's office.
Explore the library.
Find the principal.
Leave the office building.
Finish the prologue.
Kill 50 monsters hand-to-hand.