I need some help picking a final major attribute for my new custom class. I will apply this class to my char Figgin. He is a thief/assassin but not a true assassin yet. The class name is Spectre. Below is what I have so far.

Illusion
Destruction
Alteration (instead of security for locks)
Sneak
Blade
Light Armor

The last attribute is between:
Alchemy- poisons
Marksman-Different weapon but I kinda wanted one weapon to be proficient in.
Mercantile- so I don't need a fence to sell stolen goods.

Looking to be a stealthy magical theif assassin, like a Spectre. Please help me with this last attribute.

Where have you gone, Renee?