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