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