If you watch the E3 campaign demo you will here cortana say “chief” and you can see an image in chief’s visor. I think its the warden eternal. Chief’s Visor
You can see that the image has the promethean sword/spike thing like the Warden Eternal.
It’s Cortana by the looks. Shape matches and let’s not forget it has been stated that Chief, during mission 2, sees cortana’s shape while she talks about the Meridian, and that reclamation’s coming.
It’s part of the cutscene in mission 2 where Chief sees a silhouette of Cortana. That is said silhouette.
I figured it was mission 2 as well. But didnt Game informer say that chief fell in the building. I dont think he was outside in the mission. In the video you do see a blue orb that chief follows but maybe he meets the Warden Eternal and he explains things to chief. There are so many alliances in question hahaha