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