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