From Destinypedia, the Destiny wiki
The Arkborn are a race of sapient Arc-based energy beings. In their natural state, they reside in arks, interstellar conduits that dwarf The Leviathan, which they cannot normally leave. Their exotic biology may suggest a link to the Light, or possibly the Darkness.
Emperor Calus threatened the Arkborn and their interstellar conduits during his exile aboard the Leviathan. One of them, known as The Fulminator, agreed to join him as a Shadow both to keep him away from her people and in exchange for a specially engineered containment suit that would allow her to move freely beyond her ark. Her departure outraged the rest of the Arkborn.
Before he arrived in the Solar System, Calus broke his promise to the Fulminator. He returned to the interstellar conduits and, with the aid of his Councilors, kidnapped many of the Arkborn, imprisoning them within a section of his Menagerie dedicated to the very Shadow who tried to protect them from him. 
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