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