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