When I first saw the ghost, I didn't understand why it was called that. As far as I could tell it was just an A.I. companion that would turn on lights. Then people started saying that the "death orb" we all saw was actually the ghost. Then it clicked. It's called a ghost cause when you die that's what's left to revive you. Yeah, I just thought you guys should know that.
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It's actually subliminal advertising for Call of Duty: Ghosts from Activision!