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