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