Demon Altar

Demon Altars are used to create items that summon bosses. For example, the player can use Suspicious Looking Eye to spawn Eye of Cthulhu and Worm Food to spawn Eater of Worlds. They are also used for the creation of the Night's Edge.

They are found at the bottom of chasms or near them in corrupted areas as well as in some caves (in caves, they spawn at 8 feet above), and can either spawn as single Altars or in groups of two or more. Demon Altars also occasionally spawn near the original spawn point underground.

They are common in The Corruption but rare outside of it. They have been seen outside The Corruption in bunches of 1-4.

Trivia

 * It is possible to have 3 Altars spawn directly below your spawn point, completely visible without moving from the spawn.
 * In Alpha the Demon Altars were used to make certain armors.