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