If any of you have seen More Consoles recent video on Halo 5 Guardians, you will have seen that in the menu screen the first mission was called "The Timeless One", which was a nickname for the Primordial of Harum Chakor. [spoiler]I don't think the menu screen or any of its content is confirmed.[/spoiler]
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If that were true I'd be shaking with so much excitement I wouldn't be able to type.