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