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