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