The Black Demon Lightning Sea slowly faded from existence over time, and within half an hour, it'd been completely replaced by a swirling black mass.
This mass was none other than the Void Mana Damien unleashed to devour the sea. Now, it has eaten its fill and finished its job. The Black Demon Lightning space lost its support and collapsed.
But Damien's plans didn't end there, the portal he set up was a separate matter. Once he collapsed the Mystic Realm, Atticus would be sent back to the entrance he used to access it. Naturally, he couldn't allow this.
The portal's use was to bring Atticus back to the Dawn World with him. Collapsing the Mystic Realm, on the other hand, was to enter the scene with a bang.