Hala, in the Asgard home galaxy, was the first planet settled by the Asgard, and was chosen by the Asgard High Council as the location to trap and contain the Replicators. Most of the Asgard inhabitants were evacuated from their homeworld to Orilla, and a great battle was fought. The Asgard plan to trap the Replicators in a time dilation bubble failed, however, and Thor sought SG-1's help to fly the Prometheus through Replicator-dominated space and to repair and reactivate the time dilation device.
When SG-1 arrived, Hala was an unwelcoming environment. Only a single structure remained, the entire planet's surface was paved with Replicator blocks, the atmosphere was filled with electrical storms, and the Replicators had taken human form. SG-1 reset the device, and Hala and the Replicators were trapped within a time dilation field that surrounded the planet.
The Asgard realized that the time dilation field was only a temporary measure, and two years later, in an attempt to permanently eliminate the Replicator threat, the Asgard collapsed Hala's sun by artificially increasing its gravitational field, creating a black hole which destroyed the planet. However, the Replicators appeared to use the time dilation device to counteract the gravitational effects that should have drawn them into the black hole. A large number of Replicator blocks managed to come together to form a coherent mass before reaching the event horizon. The Replicators, under the control of Fifth, were able to escape, and next threatened the new Asgard homeworld of Orilla.
Cross Reference: Asgard, Black Hole, Fifth, First, Ida, Orilla, Othala, Prometheus, Replicators, Thor, Time Dilation Device
Episode Reference: Unnatural Selection, New Order