Chaos Ball

Chaos Balls, also known as Shadow Orbs, are found deep in the crevices of Corruption. To get to them, you need to dig through Ebonstone, which is possible by using Demonite or higher tier pickaxes, exploding it with Dynamite, or converting it to normal stone with Purification Powder.

Chaos Balls drop items upon destroying them with hammer or dynamite, these items are: Ball 'O Hurt, Musket, Orb of Light, Vilethorn, Whoopie Cushion and Band of Starpower. (This list might be incomplete but this definitely comes out of it.)

On destroying first Chaos Ball, a message "A chill runs down your spine..." appears. On destroying second Chaos Ball, a message "Screams echo around you..." appears. On destroying third Chaos Ball Eater of Worlds is spawned and destroyed balls count is reset.

It's speculated that Goblin Army and Eye of Cthulhu can also spawn when Ball is broken.