I apologize for the low quality, but that’s how it was in the video.
Anyway, I believe that the dark figure in this picture is a new enemy. Why? In the segment of the video that this picture is seen, most of the humans are falling backward, as if they had just been killed, and Chief’s shields are down. While this is in no way proof, something happens in this scene, and this dark figure has something to do with it. It’s possible that he/she is another Spartan, and Infinity is simply shaking.
I rather go with your infinity shaking theory.
could be the didact
> could be the didact
I think that this takes place earlier in the campaign. If you really want to know why see here.