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