Ok, I'm just gonna skim over what other people have posted real quick and sum it up in one post, then post what I'm fairly certain is going on.
The reason as to why Mendicant Bias and Offensive Bias duking it out is unrealistic is because Mendicant has walked the path of redemption for his (it's?) actions, and is also fragmented, with part of him on High Charity and part existing in the Terminals in Halo 3. Offensive Bias is most likely dead, as he was designed as a "I'm going to die, so let's make it crowded in hell" kind of tactic. If he isn't dead, then rampancy is a high possibility, as most (possibly all but I'm not certain) forerunner AI's are rampant.
As for my explanation of what is going on, watch this video:
http://www.youtube.com/watch?v=5N-CRrP9xjQ
At 7:03 a symbol appears, and slowly begins to rotate. take this symbol at it's initial position, and compare it to this symbol
http://ascendantjustice.com/files/voc/followup/didact.png
That is an extremely close match, isn't it? It is the symbol of the Didact, the forerunner responsible for the creation of the Halo Array. the Didact is often referred to as a "Human Hater" by the people who analyze the lore, possibly due to the fact that his lover is the Librarian, a forerunner who was trapped away from a safe location upon the firing of the Halos due to her study of the Human race.
The turning of the symbol could also be interpreted as the change in personality of the Didact. A more aggressive change.
It is my belief that the Didact, an AI that is based off of the neural pathways of the Didact, or an AI that mimics the Didact is the leader of the Prometheans, due to the Didact being a military presence among the Forerunners, The Didact's symbol appearing in an official trailer, and the Didact being, as a whole, considered anti-Human by lore oriented fans, possibly giving him reason to hate the "Reclaimer", or to have an ulterior motive against the chosen race to be "Reclaimer".
Just my thoughts, feel free to argue it, but I will give a rebuttal to the best of my knowledge.