Posted at IGN:
The unknown planet seen during the Legendary ending of Halo 3 and also seen in the Concept Art Trailer at Halo Fest is called Requiem. It is surrounded by a Dyson Sphere, a superstructure that contains it wholly. The planet’s giant ring-like developed areas do not resemble Human or Covenant technology because they belong to a new enemy race.
Based largely on the concept art, much speculation has surrounded the extinct Forerunners and their existing superstructures like the Halo Array and the Ark. The rings and spires match known Forerunner architectural style.
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Note from HD Admin: Want to learn more? Read about the Halo Array here and The Ark here, compliments of Halo Nation.