Renaming by Jacob: Billy's pack undergoes changes due to the birth of Renesmee Cullen, a half-vampire, half-human child. Jacob Black imprints on Renesmee, creating a deep emotional bond that leads him to rename Billy. While the specific new name is not explicitly mentioned in the book or movie, it's understood to be related to the connection between Jacob and Renesmee.
Transformation into Newborn: As a result of Renesmee's birth, the Volturi, a powerful vampire coven, believe that the Cullens have created an immortal child, which is forbidden in the vampire world. This leads to the Volturi's pursuit of the Cullens, resulting in a confrontation between the two factions. During this conflict, Billy undergoes a transformation into a newborn vampire.
Battle of Forks: Billy participates in the battle against the Volturi and their allies alongside the Cullens and their allies. He uses his newfound vampire abilities, including enhanced speed and strength, to aid in the defense of his pack and friends.
Death and Reanimation: In the final resolution of the conflict, Billy sacrifices himself to protect the young vampire Nahuel, who possesses unique telepathic abilities. Despite his sacrifice, Billy is later revived by the vampire Jane through the use of her special vampire power.
Returning to the Pack: After his resurrection, Billy returns to his werewolf pack, reassuming his role as an elder and advisor. He also rekindles his relationship with his son, Jacob, and continues to support and guide him in his new role as a protector of Renesmee.