Mantle's Approach

Are you looking for Didact's ship, the unnamed ship the Didact used in Halo: Cryptum?

Mantle's Approach is the name of the Ur-Didact's flagship in Halo 4.

History
The Mantle's Approach was the Ur-Didact's ship prior to the firing of the Halo Array. This ship was used to transport his Prometheans, and later his Promethean Knights. After imprisoning the Didact, The Librarian left the Mantle's Approach in Requiem.

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On Requiem, the Didact was ready to enact his plans shortly after his awakening; however, the one thing that he still needed was the Composer. As the Master Chief and Cortana's plan to prevent the Didact from escaping the planet failed due to Cortana's rampancy, his Cryptum flew into the massive Forerunner capital ship, before it emerged from the planet with a score of Liches and a dozen Phantoms accompanying it. Mantle's Approach then entered slipspace to Ivanoff Research Station in orbit over Installation 03.

Despite the efforts of Cortana and the Master Chief, the Didact was able to capture the Composer and test it on the station's crew, composing all humans on board with the exception of the Master Chief. The Mantle's Approach then entered slipspace once more to travel to Earth, but was followed by the Chief and Cortana in a Broadsword. It managed to hold off the and Earth's Orbital Defense Platforms for a short time before the Broadsword was able to disable four of its particle cannons, allowing the UNSC ship to blow a hole in the hull of the Mantle's Approach, allowing Chief and Cortana to enter the ship.

Fighting their way through countless Prometheans, Master Chief and Cortana were able to reach the Didact and the Composer, though not before the Forerunner fired the weapon, composing the population of New Phoenix. After a brief confrontation between the Master Chief and the Didact, the Chief was able to detonate a HAVOK nuke, destroying the Composer and Mantle's Approach while Cortana used the last of her energy to create a hardlight shield in order to protect the Master Chief from the blast.

Trivia

 * According to Awakening: The Art of Halo 4, the Mantle's Approach is large enough to have a thin atmosphere form on its outer hull.