The Eater of Worlds will start to flee underground if the player leaves the Corruption biome, but will not immediately despawn.
To spawn the Eater of Worlds, destroy three Shadow Orbs with any Hammer or make Worm Food with fifty Vile Powder and fifteen Rotten Chunks. The Eater of Worlds will (hopefully) get you the Corrupt Key.
“A massive worm who dwells in the corruption.” The Eater of Worlds is a pre-Hardmode Corruption-themed worm boss. It can be summoned when three Shadow Orbs are destroyed in a Corruption world, or when a player uses Worm Food anywhere in the Corruption or Underground Corruption biome.
Spawn. The Eater of Worlds can be summoned by crafting Worm Food at a Demon Altar, or by destroying three Shadow Orbs. It cannot spawn naturally like the Eye of Cthulhu, or some of the other bosses. It can only be summoned in The Corruption.
The player must be in The Corruption to summon the Eater of Worlds; moving too far away from the biome during the battle will cause it to flee.
Would recommend having a whole set of gold armor to get the set bonus. Have the eye of cthulu defeated so you can acquire the war axe of the night. Does 21 damage. A harpoon and a grappling hook are also useful.
The Eater of Worlds is summoned by smashing three Shadow Orbs, so you are bound to be fighting it in those Chasms regardless. Those chasms are designed for you to be fighting the Eater of Worlds there as well. Demon Altars pretty much always spawn in those Chasms, I think more so than Shadow Orbs.