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