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 5 skulls
Get into the Prison
Find Zhandul's Orb
Find Dismantler
Find entrance to the Vault of Dismantler
Solve the Checkered room puzzle
Collect and wear full set of chitin armor (4 pieces)
Survive the invasion of white blobs
Jump 25 times into a pit
Cast 500 spells
Mix 50 potions
Perform 500 ranged weapon attacks
Find 25 secrets
Kill 250 monsters
Kill your first ogre
Travel 10000 tiles
Kill a monster by dropping or teleporting on it
Cast 250 Spells
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
Find the pie
Give Toorum a hand
Backstab a monster
Open all iron doors
Collect and wear lurker's wardrobe (4 pieces)
Collect and wear full set of plate mail (5 pieces)
Collect and wear full set of Valor armor (6pieces)
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