Grandfather Mo had stayed behind precisely because all of the Mo Family's ancestors had already gone, ensuring that the defeat in the Demon Realm would not deal too severe a blow to the family's standing there.
Having also been invited by the Demon King, he easily figured out how the Demon King had harmed those Great Vehicle Stage Demon Cultivators.
The Demonic Pills and heavenly treasures used on the battlefield back then had all been provided by the Demon King, and the Great Vehicle Stage Demon Cultivators had needed them for their cultivation as well.
Chapter 316