I think the object in the Halo 4 E3 demo, rising from behind Master Chief was a war sphinx, and here is why:
Here are some excerpts from Halo Cryptum on pages 64 & 65:
First of all to explain the shape:
> From their frozen circle, three of the machines had unfolded great curved legs and now walked over the low black wall.
This would explain the spherical shape and symmetry.
Then the size:
> Then, the sphinx stepped over us and descended into the pit.
> They carried the Didact right over us.
This would explain the massive size, as the are capable of easily walking over the humans and Bornstellar, and how tall was he?
And also War Sphinxes were capable of moving at very high speeds, as shown in demo.
Cortana also said “he” as refering to object, maybe knowing what it was.
The color:
Yes usually War Sphinxes have blue coloring, not red, but as we all know when Forerunner AI type objects turn evil the go red, as in the Promethean knights and even Guilty Spark. Also the Didact symbol is usually blue not red…
So yes, this does make you lead onto that the Didact is evil, as tons of things hint this…
And if you think its a Cryptum:
Well it is very much possible, as the expcert from Cryptum on page 64 also:
> The ovoid(of a solid or a three-dimensional surface;Egg-shaped.) vessel had parted in 3 sections above its midline belt. The section had opened outward like the protecting calyx that falls to unveil a flower.
The image here is probably a more suitable as a cryptum, much smaller, but fits the quoted description above. Here.
But it is much smaller as it could be the Cryptum and the bigger object a War Sphinx.
There is a problem though both the objects size variety, the object in the demo was much larger then the screenshot, maybe it was because of the camera angle zoomed out in the demo or what, if that is true they are probably the same object.
So I will leave it at that, and what do you guys think about it?