Here is the full list of all 33 Styx: Master of Shadows achievements worth 1,000 gamerscore.
Finish Mission 1
Unlock 20 doors
Kill an enemy by poisoning the food
Extinguish 30 torches
Kill 50 enemies without making noise
No body found during a mission
Finish the introduction
Kill an enemy with an aerial kill
Drink 20 vials of amber
Kill two or more enemies by making a chandelier drop on them
Create 30 clones
Let 15 clones die
Kill an enemy who was bound by a clone
Kill an enemy with a covered kill
Give an orc one of your clones to eat
Gather all the treasures
Kill a torturer in a duel
Kill 20 enemies in duel
You looked at the World Tree a bit too long …
Eliminate a guard with a booby trapped clone
Finish Mission 2
Finish Mission 3
Finish Mission 4
Finish Mission 5
Finish Mission 6
Finish Mission 7
Pick pocket 15 guards
Unlock all skills
Do not use any item during a mission
Kill more than 40 guards with throwing knives
Eliminate 200 enemies
Unlock the Insignia of the Shadow for each mission
Manage to disable three guards, or more, inside a clone's smoke bomb