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