I have a suggestion for your Ultraman Z x AOT story:
Celebro himself created "The Source of All Living Matter" (from Attack on Titan), using it and the Power of the Titans for his "Civilization Self-Destruction Game" on Earth, and after the Power of the Titans is ended (with every Titan turning back to normal for good), Celebro reveals all of his machinations to everyone, and takes the Source as his new host, transforming it into Destrudos to complete said game, while also having the destruction of all sentient life on Earth serve as a demonstration to the Land of Light, which he plans to destroy afterwards, to ensure that the Ultras can never interfere with him and his games in the future.
I believe this would work extremely well in your story, but what do you think?