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