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