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 'Hard' mode.
Finish the game in 'Special' mode.
Kill 150 monsters with a firearm.
Kill 3 monsters with a single light grenade.
Finish the game.
Every playable character survived at Leafmore.
Finish the game in 'Normal' mode.
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.
Find the principal.
Explore the library.
Leave the office building.
Finish the prologue.