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