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