Sigma Octanus IV

Summary
Sigma Octanus IV, located in the Sigma Octanus star system is an out-of-the-way colony planet that came under attack in July of 2552. The planet's main population center, Côte d'Azur, was attacked by Covenant ground forces, and the Spartans were deployed to find out why the Covenant were so interested in the city. It turns out that the museum of the city held a Forerunner artifact that the Covenant wanted and the Spartans were able to use their distraction to level a Covenant Crusier with a HAVOK nuke. Later on, it is revealed that the Forerunner artifact contained directions to Installation 04.

To this day, it is the only human-controlled world, with the exception of Earth, that has survived a Covenant Assault. Though it is arguable that the only reason these places survived is that the Covenant were not attempting to destroy. The assault on Sigma Octanus was a distraction to remove the Forerunner artifact, and it seems likely that Regret was looking for Forerunner installations (The Ark most likely) on Earth, rather than bringing an invasion fleet.

For complete information on the attack, see The Battle of Sigma Octanus IV.