A Covenant Reforged
The Covenant have been a long-running enemy of the series since the very first game. They’ve gone from terrifying foe to kinda funny and back again, and after ending the war with them in Halo 3, you’d think they’d be gone for good. Halo 4 showed that a splinter group had returned under the Covenant branding, led by Jul ‘Mdama, an opportunist who later called branded himself “the Didact’s Hand.” The Spartan-IVs fought against Jul and the Covenant for months, even taking advantage of another opportunist who wanted to kill Jul and take control of the Covenant for himself.
Jul and the Covenant finally bite the dust in Halo 5 thanks to the combined efforts of Fireteam Osiris and the Arbiter’s Swords of Sanghelios. While they’re out of the picture, the Prometheans and Cortana very much aren’t. The UNSC may have heavy hitters like Blue Team, Osiris, and the Swords on their side, but all of that can’t entirely compare to the might of multiple Guardians and AIs under Cortana’s control.
In the comics, the Sangheili and the Brutes manage to come together, despite massive differences (like that whole Civil War thing). Cortana herself sends out a galaxy-wide message calling out the various alien races, and several Covenant are among them. Maybe they all unite under her banner to avoid dying? Failing that, maybe Arbiter can work his charm and bring them all back together to take down Cortana and her Guardians? It sounds impossible, but long odds, play big, right?
