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