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