Harmonic Convergence is a supernatural phenomenon that occurs once every ten thousand years when the planets align, causing the northern and southern spirit portals to connect in an arch and spiritual energy to greatly amplify. A human who is temporarily bonded with a spirit can make that bond permanently upon touching the portal arch during Harmonic Convergence. This is how Wan fused with Raava to become the first Avatar, how Unalaq fused with Vaatu to become the first Dark Avatar, and how Korra fused with Raava as well.