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