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