Here is the full list of all 40 Legend of Grimrock achievements. It takes around 35-40 hours to unlock all of the achievements on Windows.
Backstab a monster
Complete the game with old school mode
Mix 50 potions
Finish first level in under 4 minutes
Complete the game with hard difficulty setting
Find all treasures
Find all secrets
Find 5 skulls
Survive the slimes
Collect and wear full set of Valor armor (6pieces)
Complete the game
Collect and wear full set of plate mail (5 pieces)
Give Toorum a hand
Get into the Prison
Find 50 secrets
Open all iron doors
Cast 500 spells
Obtain level 50 in any skill
Find Zhandul's Orb
Collect and wear lurker's wardrobe (4 pieces)
Solve the Checkered room puzzle
Find Dismantler
Find the pie
Find entrance to the Vault of Dismantler
Collect and wear full set of chitin armor (4 pieces)
Survive the invasion of white blobs
Jump 25 times into a pit
Kill a monster by dropping or teleporting on it
Kill 250 monsters
Find 25 secrets
Travel 10000 tiles
Kill your first ogre
Perform 500 ranged weapon attacks
Throw a rock 100 times
Cast 250 Spells
Perform 500 melee weapon attacks
Perform 250 unarmed attacks
Cast 25 spells
Find 10 secrets
Mix 10 potions