Recently, 343 released its second trailer for the upcoming Halo Wars 2 during the Game Awards. The trailer focuses on the main antagonist and the Brute commander of the Banished, Atriox. It starts with Isabel, the new logistics AI found by Red Team on the Ark telling them about Atriox's past and how he came to be the ruthless leader of his faction that he is today - "And so battle by battle, war by war, Atriox was born." I'll let the trailer do the storytelling itself:
Halo Wars 2 will be releasing February 2017 and if you buy the Ultimate Edition you get Halo Wars Definitive Edition for free which they also released a trailer for.
And here are some notables I found in my frame-by-frame watch of the HW2 trailer: