Oshalf

One must not forget that the Fragment of the Universe Foundation inside the Underworld Relic also has part of the Soul Gem System inside it. Of course, no one other than the twins' group knows about it at the moment. Kafan was aware of it since the twins' told him, but he wouldn't mention that part since the system was still a secret. (Chapter 1999)

"In the end, the Fragment of the Universe Foundation was the only thing really capable of powering the Relic, so we sealed it inside. As for the Living Soul Devil information, I shared it with Black Locust and the other Semi-Celestials from the Underworld. We didn't know what to expect from a Living Soul Devil, but since normal Soul Devils were already such a pain in the ass, we never gave it a try... until the war in the Realm of Gods happened. Of course, by then, I was already on the other side of the Universe, so I have nothing to do with its creation."