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