The quick answer is, I don't know. I could attempt to speculate, but that would not be especially useful to you. It would be best to wait for Raze to return: if there's anything Raze doesn't know about the Divinity universe, it's probably not worth knowing.
A very simplistic (and potentially inaccurate) answer is that it's a powerful form of magic that permeates Rivellon. Its practitioners have been variously celebrated and condemned depending on the politics of the day.
