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