We'll find out what it is tomorrow.
Xbox's YouTube channel put this up yesterday, linking over to this tumblr page with a countdown that expires on Sunday evening. Press play to watch a Spartan helmet get a-sploded by a sniper round.
The biggest clue is the fact this sniper round is stamped with the word "traitor." However, we've already been tipped off to the fact that Master Chief will be on the loose as a rogue officer in this story, and Jameson Locke, the protagonist of the Halo: Nightfall live action series, is hunting for him.
The clock runs out at 6 p.m. on Sunday. The latest news we have out of Halo 5 is that Cortana — the AI and not the digital assistant on Windows phones — will return. That's per the actor for Locke himself.