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