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