The Black Ark is a gargantuan Hive ship belonging to Crevox. During the collapse of humanity, the ship crash landed on Oberon, the moon of Uranus. Since then, it has largely been entombed under ice and rock until it's rediscovery by Mia Preston's fireteam.
Comparable to a hive Dreadnaught, the Black Ark is in fact a completely different design. This classification of ship is known as the Hive Catacomb, more a fortified troop transport than a battleship. It's physical appearance is that of a black metal triangular prism, with a massive arc emitter built into the underside.
Besides the Black Ark, no other Catacombs have been discovered in our solar system.