From Halopedia, the Halo wiki
A Guardian was left by the Forerunners under Csurdon Sea and was eventually discovered by Sangheili dive-hunters and fishing skiffs. Centuries later, the Sangheili built the city of Sunaion over the sea to observe and worship the construct, which they recognized belonged to their gods, the Forerunners.
During the Blooding Years in 2558, Jul 'Mdama's Covenant used Sunaion as a staging point to launch an attack against the Swords of Sanghelios on the state of Vadam on the nearby Yermo. However, Arbiter Thel 'Vadam learned of this plan and had his forces establish forward operating base in the ruins of Nuusra on Qivro at the shores of the Csurdon Sea.
The Swords of Sanghelios, aided by Fireteam Osiris, then launched an attack on Sunaion. The battle and the Guardian's awakening heavily damaged the city. After the Sangheili refused to surrender to the Created, Cortana sent another Guardian back to the planet to pacify it. Spartan Sarah Palmer would later tell Blue Team and Fireteam Osiris that the destruction wrought by the Guardian left the remaining Covenant ships crashed in the Csurdon Sea.
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