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.
Finished mission 1
Eliminate 200 enemies
Kill an enemy by poisoning the food
Unlock 20 doors
Kill two or more enemies by making a chandelier drop on them
Kill an enemy with a covered kill
Kill 50 enemies without making noise
Kill 20 enemies in duel
Finish the introduction
Extinguish 30 torches
Finished mission 2
Finished mission 3
Finished mission 4
Finished mission 5
Finished mission 6
Finished mission 7
Give an orc one of your clones to eat
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 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
Pick pocket 15 guards
Create 30 clones
Kill more than 40 guards with throwing knives
Drink 20 vials of amber
Do not use any item during a mission
Let 15 clones die
Unlock the Insignia of the Shadow for each mission
You looked at the World Tree a bit too long...