The idea of the Black Wulf being discovered in the castle catacombs quickly led me to the question of "why was it stashed away?"
Project Cataclysm. In the unlikely event that Voltron was released and failed, the Black Wulf would be sent out. A final failsafe/last resort, basically. A soldier that doesn't need food, water or air to survive, one that can heal at unnatural speeds and doggedly pursue enemies across the universe until they break or stop to rest, systematically hunting members of the Galra down.
Sort of an Altean Super Soldier program, but worse. And the only test subject to survive.
Wulf is stashed in the catacombs under the hope and prayer of never needing to be deployed, and then forgotten, and Danny gets a nice 10,000 years of pain free, uninterrupted sleep.