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