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