Arcane Magic in the present is a real accomplishment of the Planar Council. With so many planes that have different beliefs, practices, and philosophies, developing a condensed single process originally seemed impossible.
While some ideas end up getting fleshed out to semi-completion, an extraordinary amount never get anywhere near that point. One of those ideas was… what would happen if fleas fed on the blood of a god?
The Laghairt-bonn were able to evolve due to the mistakes made by the previous masters of their world. The Seannathair, the ones who made that mistake, still exist and actively hunt them down with a hatred grown over millions of years.