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