During the H4 reveal at E3 when they show john is ambushed by above. The knight's face? opens and you can see a skull like face .
I'm a total noob at halo facts :(
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At first I thought they might be Prometheans themselves. But I now think that they are the tools of Prometheans. Maybe Autonomous Drones. From what I can gather, these are constructs of the Promethean, not the Prometheans themselves.