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