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