Ring of fire

My will carried fire to the gasoline in the circle, a single small static spark of eye-searing brightness, and flame leapt up with a low sound like something huge taking a deep breath, the fire racing swiftly around the circle’s exterior and burning with a clear, cold blue light.
Working magic inside of circles is intense. Doing it in a ring of fire, where the flames close the circle is … like being inside a room where the walls and floors and ceilings are all sheets of mirror, with infinities of reflection spinning in every direction. Anything you do with magic inside a ring of fire has a tendency to build power very, very rapidly, and to send fragments of energy rebounding around inside of it, recombining in potent and unpredictable ways—so potent and unpredictable, in fact, that while the technique was not black magic per se, it was nonetheless on the list of prescribed spells and actions that the Wardens used to assess the warlock potential of any given wizard. It was that dangerous.
Think of a ring of fire as, oh, an experimental fusion reactor. One way or another, something big is going to happen. If this banishing got out of hand, something closely resembling a small nuke could go off in the middle of the Gold Coast.

Peace Talks Chapter 12, Page 107-108