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