It consists of a flat area surrounded by seven stone pillars, all covered with lines and geometric figures. In use, all of these lines glowed red-hot. The ritual required a mortal sacrifice, and so the center of the circle also included a set of manacles to hold the luckless victim in place while he burned.
Interestingly, the circle gave satisfactory performance despite significant pre-existing damage. However, when angels, under Balthazar's command, attacked the circle while it was active, the relatively minor damage they caused inflicted instant and severe injury on Wrath and then shattered the circle's power entirely.
Despite the requirement of a sacrifice, the ritual can only last as long as the sacrifice survives. In fact, the ritual may have kept its victim alive long after he should have burned to death.
When Wrath first arrived, several of the cultists condicting the ritual were disintegrated, possibly because the circle was damaged. One of the commanders present even wondered aloud if that was supposed to happen.
Many things remain unknown about the circle. Among them are what it takes to create a circle, and whether magic circles can be used for other things.