The Great Circle

The great circle is a powerful cooperative spell that can be used only by four or more priests, each casting the spell simultaneously. Because of the nature of this spell and its casting time, it is often used to cleanse grounds in preparation for the construction of a temple or sanctuary.
When casting the great circle, the priests stand in a circle of no more than 20-foot diameter. Each faces inward; when the spell is completed, each priest faces outward, directing the energy of the spell.
When the casting is complete, the spell takes the form of a radiant halo of golden light 20 feet above the ground. This halo quickly expands in a shimmering wave. It can pass through objects, with small arcs of the halo disappearing momentarily and reappearing on the far side. As the halo moves, it generates a high-pitched hum that varies in pitch, almost like a chorus. The halo moves slowly at first, but builds speed, reaching its maximum range at the end of one round.
The radius of the golden halo is dependent on the number of priests casting the spell. Each priest adds 60 feet to the radius. Thus, four priests could generate a halo that extends 240 feet in all directions from the circle of priests. Theoretically, there is no limit to the number of priests who may contribute to this spell, but the need for the priests to be within a 20-foot diameter circle sets a practical limit of 20 casters.
The halo is pure energy tapped from the Positive Material plane. It causes harm to undead and evil beings within the area of effect. Undead creatures of 8 or fewer hit dice are instantly destroyed and are not allowed a saving throw to avoid the effect. More powerful undead suffer 1d8 points of damage per caster. A successful saving throw vs. death magic reduces this damage to half. Creatures of evil alignment suffer 1d6 points of damage per caster (a saving throw is allowed for half-damage).

Reverse: The Black Circle

The reverse of this spell, the black circle, creates a ring of shimmering black energy. Paladins and priests of good alignment suffer 1d10 points of damage per priest in the circle. All other good creatures suffer 1d4 points of damage per caster. Affected creatures are allowed a saving throw vs. death magic to reduce the damage to one-half.

[Tome of Magic]

Reversible

Sphere: Sun
(Abjuration)
Level: 6
Cooperative

Range: 0
Components: V, S
Duration: 1 round
Casting Time: 6 turns
Area of Effect: Special
Saving Throw: Special