From Halopedia, the Halo wiki
During the UNSC exploration of an unidentified Forerunner installation, this vessel suffered damage, leading the rear of the vessel to explode. Parts of the left wing would fall into the sea near a beach close to a Forerunner weather station while the fore section crashed straight into the sand; its remaining wing tilting into the air. It can be assumed that the Longsword had been grounded there for quite a period of time, considering its coloring and the growth of vegetation along its wing.
This particular Longsword seems to be significantly smaller than standard variants used by the UNSC Navy. This can be seen by comparing to other vehicles on the map. This is most likely a means of using fewer bytes - all Longswords that the player could not make direct contact with in Halo 2 and 3 are significantly smaller than their interactive counterparts (eg. the Ark level crashsite).
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