The World Must Continue Evolving
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With the defeat of Dorothea and the destruction of her Dream Dungeon, a threat larger than anything humanity could had been able to handle was prevented.
If things would had been left as they were for longer than a few days, the Dream Dungeon would had expanded across the entire city, merging both Dreams and Reality together, and generating an utterly destructive landscape, where no normal civilians would had been able to survive.
And eventually, this Dream Dungeon would had kept spreading across the Country, Canada would be swallowed in a month, then America and Alaska, then the rest of South America, it would then expand across the seas, swallow Europe, Asia, and everything else…
This was one of the Demon King's plans, utilizing Dorothea's Unique Powers combined with the many "Dungeon Seeds" that Phantasmos had left behind between the distortions in space.