In an "X-Play Exclusve" video from Halo 4, 343 Industries' Kiki Wolfkill shows off Halo's old enemy, The Covenant, working together with Halo 4's new enemy, the Prometheans.
In what Wolfkill calls a "quintessential big sandbox encounter," the Covenant and Prometheans cooperate in the "Infinity" single-player mission from Halo 4. You'll see a Promethean Watcher shielding a Covenant Knight, a situation calling for heavy artillery. "The sniper doesn't just help, you kind of need it," Wolfkill says.
Check it out:
Also of note in X-Play's Halo 4 video: Mechs! In the video above we get a look at the Mantis mechs from Halo 4, vehicles you'll almost certainly get to play in multiplayer.
"It would certainly make sense, if we were to put so much energy into creating such an awesome vehicle that we would bring it into multiplayer," Wolfkill said. "That's all I'll say."
So it looks almost like Mechs in multiplayer are almost confirmed.