Noah and Night had already started to perfect the process of severing Heaven and Earth's laws. The dark matter and the absence of external obstruction also allowed the duo to take care of Castor quickly and without interruptions.
The mutated and severed energy released by Castor's corpse flowed through the dark matter and entered the black hole. Noah had no intention to create another underling. He only wanted the cultivator's power to satisfy his hunger.
A sense of peace temporarily filled Noah. His hunger vanished for an instant, but defeating another solid stage cultivator made his ambition intensify.
That event enhanced the various aspects of his law. His pride intensified, so his hunger followed. The feeling that Noah had just appeased returned stronger than ever as his aura spread past his technique.