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