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