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