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