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