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