OP is overpowered like giving your character an ability that doesn't balance out in some way.  : P
Love Eevee, but only ever really got into to the original three evolutions for some reason.  I hope Eevee gets a mega evolution that's a normal type.  Oh beat still my thundering heart would that be cool.  

Mmmm Yeah, that could work, but perhaps since your powers are weak against light the ability will only work when your in shadows?  Oh, and attacks from pokemon that are strong against dark types would hurt you more?

Speaking of which, ever wonder why dark isn't weak against fire?  you'd think that since fire cast off so much light it would be super effective against dark type pokemon.