Titled 'Artifact', the second episode is accessible through the Spartan Ops chapter select menu and introduces five new chapters and an accompanying CG cinematic.
Here's the official description and a teaser trailer:
"The mysterious artifact recovered from Requiem takes control of vital systems on board the UNSC Infinity, forcing the Spartans of Crimson and Majestic back down to the planet to look for a way to free the ship. Commander Lasky and the UNSC need you, Spartan...get back in the game!"
In our Halo 4 review we said 343's first game is "one of the best multiplayer shooters on 360 but Reach has the better Firefight and Halo 3 has the better campaign".