Worm Food is used to summon the Eater of Worlds. The item must be used in The Corruption, but it does not matter whether it is day or night. Player-made Corruption works as well (You will have to hear the Corruption music to summon it). Attempting to use it outside of The Corruption will do nothing since the update which removed the penalty; previously attempting to use it outside The Corruption would cause the player to receive 7000 damage (minus Defense), effectively killing the player.
This item is crafted using 30 Vile Powder (Which in turn requires 6 Vile Mushrooms at Placed Bottle/Alchemy Table) and 15 Rotten Chunks, a common drop from Corruption enemies. It would also require a Demon/Crimson Altar.
Using another Worm Food while the first Eater of Worlds is still alive will again cause the player to take approximately 7000 damage (minus Defense). Another worm may appear but it will disappear after the player's death; however, the first eater will stay in the map. The death message resulting from this will appear somewhat incomplete, such as "(Playername)'s entrails were ripped out.) with no death cause mentioned.
An alternative to this method is to break 3 Shadow Orbs. Most of them will spawn under layers of ebonstone, which can be mined with a Nightmare Pickaxe or better. You can also use explosives to break the ebonstone. A hammer is required to break a shadow orb.
On occasions when summoning the Eater of Worlds, it may not spawn even though the message appears that states his summoning. Players may choose to use another worm food. This action may kill the user and a message that simply states "(player) has been slain". It commonly happens if the player summons it near space (especially in hard mode, due to a Wyvern spawning), as the spawn rate of harpies interferes with the spawning of the Eater of Worlds.