Shadow Orbs are found in the corruption and if 3 are destroyed, they summon the Eater Of Worlds.

Maxx stated in Comic #183 that he "already broke a few of these while he was exploring and nothing bad ever happened". So when June accidentally broke one of them with the musket in Comic #185 , the Eater Of Worlds spawned. The fight with him lasted from Comic #186 to Comic #200

Maxx got his musket from shadow orb.