Main article: Bosses
The Eater of Worlds will appear after every third Shadow Orb is broken or after Worm Food is used. Shadow Orbs are found in the Corruption chasms, surrounded by Ebonstone, which requires a Pickaxe with 65% power or higher to mine, although explosives or Purification Powder can be used to bypass this. Once within range, Shadow Orbs can be destroyed with a Hammer. The Eater of Worlds can only be summoned in world containing Corruption, and can be summoned at any time of the day. If it manages to escape the Corruption, it will flee and disappear.
The Eater of Worlds can be a challenge even for intermediate players, as it is a large worm-type enemy that travels through tiles, and is made up of multiple sections that can each attack separately (with separate heads, no less) when they are cut off. To defeat this boss, every section must be brought down to less than two body parts to be killed (the smallest possible section is a head and a tail with no body pieces). Due to how helpful corrupted items are, this will probably be the second boss faced in worlds that have Corruption, rather than those who have Crimson.
Eater of Worlds is the only source of Shadow Scales in the game, and drops a large amount of Demonite Ore, which can also be sold for over 7 gold a stack, making this technically the best pre-hardmode boss for farming in the game.
In Expert Mode, the boss is able to shoot dark-green projectiles at you called Vile Spit, similar to those fired from Corruptors, another Corruption exclusive enemy.