The cutscene in question is supposed to bridge the gap between The Master Chief Collection and Halo 5: Guardians. It was apparently shown by accident during a Twitch stream of The Master Chief Collection. The scene clearly shows Agent Locke talking to Arbiter as they discuss “the hunt” for Master Chief. We know Locke is going to be integral to the story of Halo 5: Guardians, which makes this all the more believable. All mirrors are being taken down, so be quick. http://www.dailymotion.com/video/x29db74_246257_videogames
