The Eater of Worlds is probably the second boss you will fight in Terraria if you have a corruption world. This boss can only be spawned in the corruption, and if you have a crimson world, you will fight the Brain Of Cthulhu instead. If you leave the corruption while the boss is there, it will despawn. This boss is unique in that, unlike most worm type enemies, you have to kill each induvidual segment.It has only one attack which is running into the player. If you are in expert mode, it will also shoot vile spit at you.
To beat this boss, I would use full lead/iron armor. For accessories, I used a warding shackle and a sandstorm in a bottle. For a weapon, I used the boomstick with one stack of musket balls (for this boss, meteor shot works very well too). Also, you should bring at least 1 stack of grenades. Use the grenades until it is split in to a ton of smaller worms, then use the boomstick. Another thing that makes this boss a lot easier is a long platform about 20 blocks in the air. This allows you to see him coming and easily dodge him. I would strongly recommend having at least 200 max life before fighting him.
