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