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