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