Basically, the Hive are just hosts for parasitic worms that were rejected by the Traveler for being a morally dubious species that found the Hive and convinced them to do their bidding by promising immortality. Once the worms corrupt and have complete control of the Hive, they first convince them to destroy all other life on their planet to obtain technology to move into space, then to invade a moon around their homeworld and steal their interstellar travel technology to lead a crusade to destroy the Traveler and everything that the Traveler has influenced. The Hive have been chasing the Traveler for about 25,000 years now. Each time they find it, they wipe out entire civilizations in their quest.