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