It should be Nothing is True, Everything is Permitted (RWBY x Assassin's Creed )

Altaïr ibn la Ahad, the Grandfather and Mentor of the Syrian Brotherhood of the Order of Assassins, had finished his task of protecting the Apple when he died and, waking up in an unknown place in the middle of the forest, went to a watering hole..., he reflected that he was younger when he was just an assassin. He saw that he had his assassin's equipment, but he drew his sword to defend himself. He saw that he was surrounded by strange creatures and quickly looked up at the sky and what he saw amazed him: the moon was broken and he realized that he wasn't in his world, the moon should be full and complete, but he also wondered if this new world also had Templars, how Altair would deal with the threat in Remnant.

His mission now was to defeat Salem, another battle between light and darkness, with the hunters against the grimms and Altair in the shadows, just as the assassins had done for a long time, and save Remnant.