Mendicant Bias. As you may have know, during the forerunner-flood war, Mendicant Bias was designed to combat the flood, but he was not gifted with combat experience and was captured whilst fighting the grave mind, he mini pulsates him, corroded him and made him turn on his creators and fight WITH the flood. When the forerunners found out, they made Offensive Bias to combat Mendicant Bias. He won and scattered pieces of him all throughout the galaxy. Each piece was a piece of his personality. One was wisdom, which was found by the prophets and became "the oracle" and since he was damaged, he set their beliefs for the war to come. The one that was talking backwards was the piece of regret. He was the most whole part and was still semi sentient so roamed the galaxy seeking forgiveness. The piece of him shone in the halo 5 trailer is (my speculation of course) anger, wanting revenge and master chief knows he is the closest connection to the forerunners, his only hope for Cortana to return, he can access with his AI.