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