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