How about something equivalent to a nuke drop that killed off half the population of all supernaturals? It would me more mystical of course, and maybe even very vicious, killing off most of the children? Children deaths are always a good reason to take a step back and realize that what you are doing just isn't worth it any more. However it MUST be clear that neither side can confirm that it's the other side that did it. That way it makes more sense that they would be willing to work together rather than one side being demonized for devastating the under 12 population.