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