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