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The Awoken of the Reef were aware that the Black Garden had a heart, although they weren't certain what it actually was. Uldren Sov speculated it was "a seed left behind to grow", or a baited "tripwire" existing solely to be destroyed, alerting its creator that the Traveler had reached some critical stage in its recovery.
It is of central importance to the Sol Divisive collective, who worship it as a god, even remaking themselves in its image. In the Garden, the Black Heart appears as a massive, floating mass of chaotic fluid with energy emanating from it. When in danger, the Heart summons its vessels, the Sol Progeny, to defend itself.
In a conversation with Commander Zavala, Ikora Rey hypothesized that the Black Heart was a being so beyond comprehension that even the Vex saw no logical option other than worship it. However, the abomination was too weak to summon all three Sol Progeny at once, and the fact that it needed to summon them in the first place meant that it was vulnerable. Nonetheless, she asserted it must have been a mere portion of a greater power, probably even a fragment.
Some time after the Heart's destruction, The Undying Mind attempted to restore it by reverting the Garden back to the Darkness, but the Guardians were able to stop it before the Mind could complete its task.
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