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