SUMMARY
The Battle of Sunaion was a battle between the Swords of Sanghelios led by Thel 'Vadam, their allies Fireteam Osiris led by Jameson Locke, the last of Jul 'Mdama's Covenant forces, and the Created forces led by the Warden Eternal in the state of Sunaion on the Sangheili homeworld of Sanghelios. The conflict began as a push into the last Covenant held territory by the Swords and Osiris, both to finally defeat the Covenant forces, and for Fireteam Osiris to gain access to the Forerunner Guardian at the site. The attack was successful, though access to the Guardian was interrupted by the Warden Eternal, who attempted, but was unable to keep Fireteam Osiris from getting to the Guardian. Osiris was then sent to Genesis by the Guardian they had landed on, and became involved in the Battle of Genesis.
RELATED MEDIA
These events are depicted in the video game Halo 5: Guardians, which was the subject of a Spoilercast on November 9th, 2015.