Its Creator

A crawling creature blocked their path, looking like a giant centipede with oozing black tar. The elves released their spells, letting the elemental attacks rain down on the Centipede of Doom. However, it thrashed about and spilled the black tar everywhere. 

Diana stared at the creature and remembered what Arthur taught her about the creatures of nothingness. The black tarry substance was not actually something that belonged to this world but a spiritual manifestation of the nameless itself. Therefore, it was impossible to destroy it. 

"Nameless is akin to a poison for worlds," said Arthur on one of their dates. "It fills the worlds with corruption, leading them to downfall. However, it doesn't have a form of its own. That's the reason it's weak once the vessel is destroyed."