Dang it! I had this exact same idea last night. Good thing I checked to make sure no one beat me to the punch. I was going to call it Break of Dawn and if Nightfall was also in play neither of their effects would work but I think I like the destroy bit even more. Maybe what it should do is if Nightfall is already in play when this card is played then Nightfall is destroyed and all death/darkness creatures are stunned for one turn. And Nightfall should have the same effect if this card is already in play. That would make for some awesome back and forth if two decks based on these cards met up!
I don't think this is OP for the life or light rush decks because you can't guarantee that you will get the card early without putting multiples in your deck which really dilutes it since like Nightfall playing multiple versions does nothing extra. Even though it has good synergy with cards like frogs and cockatrice spam, and ROL; Nightfall has similar synergy with cards like skeleton spam, pests, vampires, and deathstalkers.
Anyway, awesome card idea!
Congrats on beating me to it.
