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