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