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