Chaos Ball

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 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. (This list might be incomplete but this definitely comes out of it.)

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

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