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