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