Here is the full list of all 33 Styx: Master of Shadows achievements. It takes around 25-30 hours to unlock all of the achievements on Windows.
Kill a torturer in a duel
Eliminate a guard with a booby trapped clone
Manage to disable three guards, or more, inside a clone's smoke bomb
Unlock all skills
Unlock the Insignia of the Shadow for each mission
Gather all the treasures
Kill more than 40 guards with throwing knives
Do not use any item during a mission
Let 15 clones die
Pick pocket 15 guards
Kill an enemy with an aerial kill
Drink 20 vials of amber
Kill an enemy who was bound by a clone
Kill 20 enemies in duel
Create 30 clones
Finished mission 7
Finished mission 6
Eliminate 200 enemies
Finished mission 5
Finished mission 4
Finished mission 3
Kill 50 enemies without making noise
Give an orc one of your clones to eat
Finished mission 2
Kill two or more enemies by making a chandelier drop on them
Kill an enemy with a covered kill
Finished mission 1
Unlock 20 doors
Kill an enemy by poisoning the food
Extinguish 30 torches
You looked at the World Tree a bit too long...
Finish the introduction
No body found during a mission