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