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