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