Midnight is the 8th and final level in the Halo 4 campaign.
It was first announced along with the rest of the achievements for Halo 4.
This is the only mission in Halo 4 where the F-41 Broadsword is used and where the Gravity Hammer appears, allowing players to get the Chief, Smash! achievement. (Source)
***HALO 4 ENDING SPOILER***
Epilogue is the final cutscene for Halo 4. It shows how John is saved after the end of the mission Midnight and how he is moving on after the death of Cortana. The Didact is also heard giving a speech.