This is a theory on why Halo 5 Guardians could see the return of The Flood. This comes to us courtesy of YouTuber HaloFollower, whom you can subscribe to here.
First off is based on Halo Escalation issue 6. There is a panel that shows the cryobay of Spirit of Fire, with one lone Flood infection form, surrounded by open pods. This may be a repeat of the situation in the original Halo Combat Evolved.
This is the apparent translation of the text in that listing:
These Spartan warriors, with the Master Chief as a figurehead, are the last salvation for mankind in a battle against the Flood and the Forerunners!
Of course, this could either be a leak or faked information.
Lastly, there is 05-032 Mendicant Bias, the ancient AI ancilla who sided with the Flood. It is entirely possible that Mendicant Bias was featured in the initial Halo 5 E3 teaser trailer. Mendicant Bias was buried in an unidentified desert, as its information was useful, but it could not be trusted, even though it turned sides again back to its creators. Basically, it would only be awakened if there was a need for information on the Flood.
Of course, we don’t know for sure that the thing Master Chief faced is Mendicant Bias, but if it is, it is only a confirmation of what we already suspect.
Halo 5 Guardians will be released exclusively to Xbox One in 2015.
