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