"Well," says Saevetei, searching through a shelf for something, "basically - we can use a thing called essence for magic. Everyone, not just certain people. There are naturally wells of essence in the world, and for convenience we moved them to places that were easier to reach." Search search search ah ha! She retrieves a little strange circular device that looks to be made out of some kind of glimmering silver-blue net thing, with a copper frame. "And this turned out to be a really, really bad idea, because new wells of essence - we call them nodes - formed in the empty node-less places, but without any essence themselves. So they stole it from everything in the surrounding area, and this killed everything nearby." She goes looking through another shelf that looks like it's for vials of some kind.

"To make matters worse, the nodes that were all brought to the same location turned out to, ah, eat each other when put in close quarters. And the last node doesn't stop eating essence once it's run out of fellows to eat. Which also kills things." She retrieves a vial of clear liquid. "And both of those together caused that. I'm fixing it, but it takes a while."