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