Little idea that came to my head... What if there were two dungeons on oppisite sides of the world, one on top of corrupption the other a hallow dungeon.
Now each of these Dungeons have Knights, Hallow Representing the Light side, and Corruption being Dark.
What if thats how the world of Terraria started? Each side tried to kill the other until both of them faded away, leaving trace amounts of there activity behind. There creatures they had created now ruled the land they had left.