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