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