The Seal Sphere is some sort of object that contains the essences of a number of Alimbic people. Its sole purpose was to imprison the creature Gorea. The Seal Sphere was part of the final desperate plan of the Alimbics.
Having failed to destroy Gorea, they prepared the Oubliette station to place in the Infinity Void. To keep Gorea there, they used their own spirits to create a device that would somehow bond to Gorea when it attempted to assimilate it. The device worked, and with the psychic powers of the Alimbics containing Gorea, the seal could be maintained indefinitely.
During the events of Metroid Prime Hunters, the six other Bounty Hunters ignorantly free Gorea from its main prison. When Samus fights Gorea, destroying its arms will release the Seal Sphere upward. Gorea sends a tentacle through the Sphere to attack Samus, but Samus can shoot the Sphere with whatever Affinity Weapon Gorea is currently vulnerable to. Samus manages to damage the Seal Sphere, reducing the power of Gorea, but eliminating the Alimbic presence within. In a lower chamber, Gorea changes, assuming a different form. The Sphere now has only pure energy. Gorea now remains bonded to it to draw power. When Samus destroys Gorea, the Sphere is presumably destroyed.