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