As the fourth forbidden area with an abnormally high gravity due to the concentration of space laws, the Gravity Ridge was the forbidden area that Alister chose for sealing the Demon King.
Although it might have been better to seal the Demon King in the Sea of Chaos, the Sea of Chaos had distorted the world laws in its surroundings. There was a possibility that the seal they made would be destroyed by the law of chaos.
Furthermore, Alister and Zanna might be confident that they could seal the Demon King in the law of chaos without their seal getting destroyed, but the Sea of Chaos might send the Demon King to another world.
Alister didn't want that to happen so he dared not seal the Demon King in the Sea of Chaos. The Gravity Ridge was just the best sealing spot for the Demon King so he chose it instead.
With the sealing area of the Demon King being decided, Alister and Zanna left the Extreme Volcano Range.
