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