What will happen when 2nd magic is a thing with shadow? Bc I am a shadow user and I can’t see any synergy going around with shadow

you don’t have to pick a 2nd magic. Shadow is powerful on it’s own, complementing it with a weapon/strength build would probably be one of the best options
What Mim said. Shadow is a pretty strong stand-alone magic, and you don’t really need to combine it with anything else if you don’t want to.
On the other hand, you know what would be way stronger with a weapon/strength build?
Heat magics that synergize with bleed.
Magma 
