Unidentified Forerunner colony
From Halopedia, the Halo wiki
This former Forerunner colony is a planet in the Zeta Hydronis Secundus system that was the location of various buried labyrinthine Miner installations. Naively considered an impregnable fortress by the Forerunners, the world eventually fell before the parasitic Flood.
At the height of the Forerunner ecumene's power, the Forerunner Miner rate had built a network of colonies buried deep inside the rich planetoids of the Zeta Hydronis Secundus system. Protected by layers of hard light and exotic matter, and fitted with energy projectors, the world was considered impregnable. During the Forerunner's war with the Flood, the installations successfully held back an attack from the parasite. Their victory was short-lived however, when the Gravemind itself turned an infinitesimal portion of its immense intelligence to bear on the problem.
Grasping impossible filaments buried beneath reality by the long-vanished Precursors, the Gravemind pulled at strands that twisted and warped real space. Walls made of collapsed starmatter cracked and shattered, and the planet was ripped apart. With the Miners' defenses destroyed, the Gravemind poured billions of combat forms onto the planet. Those that survived the assault could only activate failsafes and await their death at the hands of the Flood.
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