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