C3931 Victory and Defeat Had Been Decided

From the looks of it, the Plague Devil King already knew that this place wouldn't be able to hold on for much longer. That was why he allowed this trace of origin to merge into the Ruin Returning, and was automatically assimilated by the Ruin Returning Power.

Therefore, although his origin had been assimilated at this time, it hadn't completely vanished. On the contrary