From Destinypedia, the Destiny wiki
When entering the Lost Sector, the Arcadian Chord will be surrounded by an impenetrable shield. A blue Vex construct looking similar to an Oracle will hover above it, which has to be destroyed in order to remove the shield and make the Minotaur vulnerable to damage. After a certain period of dealing damage, the shield will reappear along with multiple Oracles scattered around the Sector, which when defeated will once again lower the shield. This process repeats until the Arcadian Chord is killed.
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