SUMMARY
The Battle of Cassidy III was a conflict between Spartan-IV fireteam Alpha-Nine, allied with United Rebel Front forces on Cassidy III, and the Promethean forces under command of the Created late in 2558. Alpha-Nine originally traveled to the system to investigate the reason why Cassidy III was undetectable to outsiders aside from direct observation, but became embroiled in a battle with a Guardian after the AI Leonidas tracked them to the colony. While Alpha-Nine and rebel forces were able to fight off the Guardian long enough to allow many colonists to evacuate, the planet eventually fell under Created control, submitting to their authority.
RELATED MEDIA
These events are depicted in the novel Halo: Bad Blood, which was the subject of a Halo Book Club on August 6th, 2018.