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