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