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