Kallana had been a sandy desert world, inhabited by free Jaffa bearing the tattoo of Cronus. When a Prior visited the planet, promising retribution if the people did not bow down to the Ori, the Jaffa, led by Latal and his wife Birra, refused to trade one false god for another and prepared to defend their world. However, their staff weapons had no effect on the Prior, who used his powers to fling them far into the distance and to raise a bluish force field around the area of the stargate. As more Jaffa fired their weapons, the force field grew in size and strength, and the Prior remained near the active stargate, despite the highly toxic environment inside the field.
As the force field expanded, the stargate remained active for more than two days, suggesting that it was being powered at the opposite end by a black hole, the only power source capable of maintaining an active wormhole for more than 38 minutes. On the advice of Nerus, a Goa'uld who had secretly allied himself with the Ori, SG-1 gated to P4C-452 to rendezvous with the Prometheus in an attempt to destroy the Kallana force field using a Mark IX nuclear warhead. The force field expanded in predictable increments, about 30 minutes apart and lasting 16 seconds each. During an expansion phase, SG-1 was able to beam inside the force field, protected by EVA suits, and they ordered the Prior to disengage the stargate within 30 "Earth minutes." When he failed to comply, they detonated the warhead, and an arriving Jaffa fleet under Gerak's command continued to fire at the planet, however in doing so they inadvertently provided the power required for the force field to envelope the entire planet.
With the force field complete, large rectangular objects began coming through the stargate and rising from the planet's surface, arranging themselves into a circular shape above Kallana as the planet rapidly began to collapse into a micro-singularity. The individual units, like links in a chain, formed an enormous stargate, dubbed a "supergate" by Carter. The supergate, powered by a black hole and large enough to accommodate the passage of an entire armada, would allow the Ori to maintain an open wormhole indefinitely to span the vast distance between galaxies and establish a beachhead for invasion. However, Vala's cargo ship disrupted the final formation of the ring, and the shock wave created by the formation of the singularity destroyed the supergate, leaving only a black hole where Kallana had been.
Episode Reference: Beachhead
Stargate Coordinates: 6-16-8-3-26-25