This unidentified vessel is a ship of unknown type used by the UNSC Navy.
This class of ship was stationed at Reach during the planet's fall. During the UNSC Pillar of Autumn's escape it flew past debris of this ship class.
The UNSC Pillar of Autumn flying through debris of this ship class during the Fall of Reach in Halo Mythos: A Guide to the Story of Halo.
Unused Halo Wars 2 concept art depicting the ship.
What appear to be these ships on an alternate cover of Halo: Tales from Slipspace.