Altars

Demon Altars are naturally-occurring crafting stations found mainly in and around Chasms in The Corruption, caves in The Crimson, and more rarely in other areas Underground. They are used to craft boss-summoning items and the Night's Edge. Altars emit some light, making them easy to spot. They cannot be freed, picked up, placed, or crafted. After a world enters Hardmode, they can be destroyed, but attempting to hit them with a hammer before that causes the player to lose half their health, and the Altar will not be affected. Whether the Altar is purple or red depends on whether it is a corruption or crimson world.



Hardmode
After the Wall of Flesh has been defeated in a world, Altars can be destroyed with the Pwnhammer or any superior hammer. Each destroyed Altar causes one type of Hardmode ore to appear in the world, with the appropriate message displaying in the players chat: "Your world has been blessed with (ore)". If a player attempts to destroy the altar with a lesser hammer, or in a world in which the Wall of Flesh is not yet defeated, the altar will be unaffected and the player will lose half their current health, as stated above.

Smashing Altars always results in Cobalt(Palladium), then Mythril(Orichalcum), then Adamantite(Titanium) ore being generated in respective order. Smashing a fourth/seventh/etc. The Altar resets the sequence after Adamantite/Titanium, and continues to cause additional Hardmode ore deposits, thus increasing the distribution of the Hardmode ores with each successive time the player smashes an Altar. Each time an Altar is destroyed, 1-3 Wraiths spawn, regardless of the time of day. When destroying multiple Altars in rapid succession, the resulting Wraith swarm may become difficult to deal with.

Which ores are generated is not determined until you smash a Altar. For example, it is not determined which ore your world has, cobalt or palladium, until the first altar has been smashed. The same goes for Mythril/Orichalcum with the second altar, and Adamantite/Titanium for the third. But once the first three altars are destroyed, the sequence is permanently set. If you get Palladium, Mythril, and Titanium, with your first three altars, your world will always produce those and only those when altars are destroyed.

Hardmode ore spawns

 * Small maps:
 * Cobalt Ore spawns below a depth of 760 feet (380 tiles)
 * Mythril Ore spawns below 1,000 depth (500 tiles)
 * Adamantite Ore spawns below 1,400 depth (700 tiles), 100 feet (50 tiles) above the depth at which Lava begins appearing.
 * Medium maps:
 * Cobalt Ore spawns below a depth of 1,100 feet (550 tiles)
 * Mythril Ore spawns below 1,400 depth (700 tiles)
 * Adamantite Ore spawns below 2,130 depth (1,065 tiles), 170 feet (85 tiles) above the depth at which Lava begins appearing.
 * Large maps:
 * Cobalt Ore spawns below a depth of 1,270 feet (635 tiles)
 * Mythril Ore spawns below 2,000 depth (1,000 tiles)
 * Adamantite Ore spawns below 2,930 depth (1,465 tiles), 420 feet (210 tiles) above the depth at which Lava begins appearing.

Tips

 * Players about to enter Hardmode may benefit by mining paths to several known Altars beforehand, as navigating Hardmode Corruption/Crimson is dangerous at first.
 * If you destroy every Altar in a world, it then becomes impossible to use them for crafting (at least within that world). Leaving one that's easily accessible in your world might be wise, there is usually one in the nearest cave from spawn at underground level. Alternatively, you can also create a new world and use Altars there for crafting.
 * Players should not destroy all the altars just for ore as the amount of ore spawned reduces for every 3 destroyed altars. And the altar, when smashed, will spawn a tile of corruption or hallow, which will soon grow into a new corruption or hallow.