Chaos Ball

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

Shadow Orbs 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.

Destorying your first Shadow Orb will relay the message "A chill runs down your spine...", and summon a Meteorite to crash into the world.

On destroying a second Shadow Orb, the message "Screams echo around you..." appears. And it's a chance that a Goblin Army appears near your spawn

On destroying a third Shadow Orb, the dreaded Eater of Worlds is spawned and the destroyed orbs count is reset.

It's speculated that Eye of Cthulhu can also spawn when an orb is broken.