As the Day of Reckoning approached, the Ori prepared to establish a wormhole between galaxies for the invasion of the Milky Way. A Prior arrived on Kallana, a Jaffa world in this galaxy, and raised a bluish force field around the area of the stargate. 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. SG-1 attempted to destroy the force field using a Mark IX nuclear warhead, but in doing so they inadvertently provided the power required for the force field to envelope the entire planet. With the force field complete, objects began coming through the stargate and rising from the planet's surface. Approximately 10 meters long and shaped as rectangular prisms, the objects accelerated and arranged 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, were held together by an energy field, and they formed a ring approximately 300 to 400 meters across, an enormous stargate dubbed a "supergate" by Carter and later referred to as "chaapa'ko" by Bra'tac. 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.
Vala attempted to prevent the formation of the supergate by maneuvering a cargo ship into position to take the place of the final link. Her plan was successful, and the cargo ship disrupted the energy field holding the units in place, however, as she prepared to ring back to the Prometheus, the shock wave of the forming singularity destroyed the supergate and drew Vala and the transport matter stream into the Ori galaxy. Following the incident at Kallana, the SGC and its allies, including the Asgard, Tok'ra, and Jaffa, continued to monitor and investigate gravitational disturbances and subspace anomalies that could indicate the presence of another supergate.
Months later, a fully formed supergate was discovered near P3Y-229. An allied fleet, including the Odyssey, the Korolev, and ships of the Jaffa, Asgard, and Tok'ra, was unable to destroy the supergate. Carter and Kvasir developed a plan to reprogram the dialing control crystal so that the gate could be dialed out before the Ori dialed in, and Carter performed an extravehicular space walk to replace the crystal. However, before she could complete the initialization of the crystal, the massive supergate was activated by the Ori, and four Ori warships entered the galaxy, annihilating the forces of the allied fleet before entering hyperspace.
The supergate disengaged and remained inactive following the arrival of the first wave of Ori ships, and subsequent attempts to block the gate by dialing out failed. Attempts to dial in also failed because the supergate is designed to dial in from another galaxy, and the energy required to create a stable wormhole is exponentially proportional to the size of the gate, thus the energy needed for a connection to the supergate far exceeds that of an ordinary stargate. Instead, the SGC devised a plan to force a wormhole arc to establish a connection between the supergate and the Pegasus Galaxy. SG-1 visited Atlantis in the Pegasus Galaxy, and with the assistance of Rodney McKay, Carter and Mitchell used the Odyssey to deliver a stargate near a black hole in the Pegasus Galaxy. Meanwhile, Teal'c deposited a secondary stargate near the supergate in the Milky Way. As the Pegasus gate was drawn into the accretion disk, the gate was dialed out and a connection was established with the secondary gate. A 26-megaton directed energy shaped charge detonated near the event horizon of the Pegasus gate was intended to cause the wormhole to arc from the secondary stargate to the supergate. The first two attempts failed, but on the third attempt, the Odyssey, under attack from the Wraith, beamed two warheads onto the Wraith ship as it was in position above the active Pegasus gate. As the Wraith ship exploded, the energy spike caused the wormhole to arc to the supergate, and an Ori warship waiting nearby was destroyed in the kawoosh as the supergate activated. Powered by the black hole, the resulting wormhole established an indefinite connection with the supergate, preventing the Ori from using it to send additional warships.
After Daniel acquired Merlin's knowledge and the ability to complete the weapon that would destroy ascended beings, he returned to SG-1 with a new plan. His intention was to send Merlin's weapon to the Ori galaxy, which would require that the wormhole to the supergate first be disengaged. SG-1 commandeered an Ori warship and completed the prototype of Merlin's weapon. In the Pegasus Galaxy, the Daedalus used a Mark IX warhead beamed directly behind the event horizon of the Pegasus gate to destroy the gate and sever the connection. Then Daniel, using his powers as a Prior, dialed the supergate to the Ori galaxy and sent the warship and the weapon through before the supergate once again disengaged. Although the plan was successful, there was no way to know if Merlin's weapon had eliminated the ascended Ori. Shortly afterward, however, the supergate once again activated, and at least six more Ori warships entered the Milky Way Galaxy, with many more likely to follow.
Cross Reference: Atlantis, Black Hole, Chaapa'ko, Crystals, Day of Reckoning, Doomsday, Kallana, Kvasir, Vala Mal Doran, Rodney McKay, Merlin's Weapon, Odyssey, Ori, Ori Warriors, Ori Warship, P3Y-229, Pegasus Galaxy, Prior, Prometheus, Stargate, Wormhole Arc, Wraith
Episode Reference: Beachhead, Camelot, Flesh And Blood, The Pegasus Project, The Shroud