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