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