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