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