Here's what I think: ok, if you have read up on your mendicant bias history it says he was destroyed, but a rampant fragment of him was lost along with instillation 07. In Halo 5 Chief is paired up with an AI that reminds him too much of cortana, before his journey to find her he he leaves the AI on the infinity. On his journey, he some how comes across instillation 07, he says it is one he has never seen before, he decides to land on it to explore and hopefully find cortana. But he doesn't, he finds something worse. Mendicant Bias and his ring-round army of flood to wipe out the human race. He flees the ring, to warn infinity of the threat, it's too late. the infinity has now been over run by flood. Chief goes to the only person he can trust now. The Arbiter, and the Sanghellii. Chief tells them that a new race of elites is leading the covenant. They join the humans once again. Then the Mendicant-flood/Human war begins. That's all I got.