Spartan John-117 destroyed Alpha Halo in 2552 by overloading the engines of the UNSC Pillar of Autumn. Upon the imminent destruction, the installation tried to jump into slipspace. This only partially succeeded with the Alpha Shard being send to the location dubbed Location 6201.
Before the UNSC Pillar of Autumn was destroyed, an automated recon drone, Recon-14, launched itself. And as it turns out, travelled with the Alpha Shard to it's final location, stuck in the gravity well of a nearby star.
The Office of Naval Intelligence cought wind of Recon-14. The explosion of the engines of the UNSC Pillar of Autumn ended up creating an entirely new element, which was illegally mined by smugglers. In turn this was shipped to Human colonies, including Sedra. After the discovery of the damage it could do against humans (and no other species), a Sangheili terrorist created and used a bioweapon based upon this element, in Sedra City.
Commander Jameson Locke, and Randall Aiken of the Sedran Colonial Guard, and both of their units, traveled to the Alpha Shard in order to stop the smugglers mining the element. Most of their team didn't make it, due to a big colony of Thanolekgolo worms living on the shard.