From Destinypedia, the Destiny wiki
The soldiers of the Broken Legion had plans to attack the Reef. However, the Cabal High Command ignored their pleas, causing the legion to massacre the commanding officers, marking them as traitors of the Cabal Empire. The Broken Legion then set a course for the Reef, but through some unknown means, they became denizens of the Prison of Elders. The legion continues to wait until they face challengers from the outside world in the hope of ascension.
Later, it was revealed that the Legion was a Trojan horse set-up by the Red Legion. Rather than letting the Reef know of this, Uldren Sov remained silent on the revelation, which allowed the Red Legion to set up a foothold in the Reef.
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