Reclaimer is the 5th level in the Halo 4 campaign. It was first announced along with the rest of the achievements for Halo 4.
Reclaimer includes the first appearance of the Mammoth vehicle, the Lich dropship, and The Librarian.
It starts off when the player is dropped off by a Pelican.
Throughout the level you will be guiding the Mammoth through the battlefield so it can destroy the cannons guarding the gravity well. (Source)