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