Genocidal Pact

The Genocidal Pact is an agreement made by Mitoki and Haruno in a flashback during the first parts of Season 2. This agreement was of a trust based bond between the two, as neither will turn on the other one. This also saw them giving the other a piece of each's essence and mend it with their own, making the full "merger" that they are one in the same.

Origins
Back in Age 670, Haruno was seen evading the law force, as he was found in possession of the Tuffles' enhanced weaponry, blueprints, and their sacred technique. As the officers were getting closer, an unknown figure would appear and mercilessly kill those officers, allowing Haruno to escape with his stolen equipment. About a week after that incident, that same figure appeared in front of Haruno. This figure, Mitoki, offered immense power to Haruno if he decided to train with him, in exchange for his unyielding loyalty to him. Realizing that there was something familiar about this man, but the offer was true, he accepted, and went off with him for about three years and trained with him.