The Battle of Beta Eridani was a Covenant assault on UNSC forces in the Beta Eridani system, shortly before the summer in 2552. The Covenant Fleet of Valiant Prudence was in search of Forerunner artifacts in the system, and stumbled upon the Beta Eridani Naval Training Range. Though there was a sizable force stationed there, all ships were crewed by cadets, and were ordered to flee the system. Any remaining forces were destroyed by the Covenant fleet.
These events are referenced in the tabletop game Halo: Fleet Battles.