I just found your card thanks to the Darkness poll. Your card is, hands down, the most innovative and most interesting. It comes with a HUGE slew of use cases.
1. This is an alternative to Gravity Pull on Voodoo Dolls.
2. Against most shields, fireflies only deal 2 damage. move that damage over to the Demon and get +10 damage along the way!
3. Similarly, you can get extra potency out of a card you were planning to Immolate anyways.
4. Delete your antimattered creatures as well as reduce their healing numbers.
5. A new and different source of death effects.
6. Power up your Phoenixes! Net benefit of 3 damage on a phoenix, or 6 damage on a minor phoenix, in addition to 2 damage for your demon.
With the wide variety of uses and effects, it's incredibly hard to imagine exactly where "balance" lies. What if it was +1|1 or +2|1? Is +5 the right number for the target creature? It's impossible for me to say, but I really hope your card makes it into the game someday. It could really shake things up and add a bunch of new deck ideas, regardless of whatever numbers Zanz ultimately chose.