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