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