The Heretic Alliance was an alliance that existed during much of the 2nd RPG. It was primarily aimed at crushing the Prophets and Brute's before their evil plan could be launched. The Heretics viewed the humans as a Neutral target, neither helping them or fighting them. This changed near the end of the 2nd RPG, when the Sangheili were casted out from the Covenant. The Heretics, allied with the majority of Elite's, helped the humans in taking the Ark. (The Portal on Earth, not the Ark itself.)