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