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