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