Microsoft unveiled the first footage for Halo: Nightfall at San Diego Comic-Con earlier today. The trailer features Agent Locke, portrayed by actor Mike Colter, leading UNSC marines on a mission on what appears to be a broken part of a Halo. It has also been confirmed by 343 Industries that Locke will be playable in some capacity in Halo 5: Guardians.
As previously revealed, Halo: Nightfall will be bundled with Halo: The Master Chief Collection, which is set to be released in November. Halo 5: Guardians currently has a fall 2015 release.
