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