Microsoft has confirmed that Halo 5: Guardians will be released on October 27th, 2015.
The platform holder revealed the game's launch date in the trailer you can see above. In the 'Spartan Locke Ad' we see Agent Locke wander through a wartorn environment towards a statue of Master Chief. Spartan 117 is lying, wounded, at the base of the statue but rather than go to help, Locke drops his assault rifle and takes out a pistol, seemingly intent on executing Chief.
At the end of the trailer, the words 'Hunt the Truth' appear on screen. Developer 343 Industries has suggested that the UNSC is hunting Chief as a traitor in Guardians.
Mike Colter, Agent Locke's voice actor, recently stated that Cortana will feature in the events of Guardians despite the ending of Halo 4.
Update: Microsoft has now released a second trailer, which sees the roles of Master Chief and Agent Locke reversed. You can watch it below.