Altars

Demon Altars and Crimson Altars are naturally-occurring crafting stations found mainly in and around Chasms in The Corruption or 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 with Hardmode-tier hammers, 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.



Hardmode
After the Wall of Flesh has been defeated in a world, Altars can be destroyed with the Pwnhammer or any superior hammer. 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. Each destroyed Altar causes one type of Hardmode ore to appear in the world, with the appropriate message displaying in the player's chat: "Your world has been blessed with (ore)".


 * 1) First Altar: Cobalt Ore or Palladium Ore, chosen randomly.
 * 2) Second Altar: Mythril Ore or Orichalcum Ore, chosen randomly.
 * 3) Third Altar: Adamantite Ore or Titanium Ore, chosen randomly.

Players will be able to find these new ores scattered across the Underground layer to near-Underworld levels, with them replacing any blocks created during world generation. They will spawn at random locations in varying chunks, in places explored and not explored alike.
 * Which ores are generated is not determined until an Altar is smashed. For example, it is not determined if the world has Cobalt or Palladium until the first Altar has been smashed. The same goes for Mythril/Orichalum with the second Altar, and Adamantite/Titanium for the third.
 * Subsequent Altars destroyed will begin the cycle again, though the fourth to sixth Altars will drop half as much ore as the first ones, the seventh to ninth will drop only one third of the ore of the first ones, and so forth. This "multiplier" can be calculated with the formula 1/(ceiling(n/3) + 1 - sgn(n)) where n is the number of Altars destroyed. So, destroying the 19th Altar would yield one-seventh of Cobalt/Palladium Ore compared to the first Altar.
 * Altars are still needed for crafting certain items, destroying every Altar in a world is not recommended.
 * Destroying more Altars will never spawn Hardmode ore types beyond the three that were randomly assigned to your world during the initial round of destroyed Altars. A world with Palladium Ore deposits will never spawn Cobalt Ore deposits, or vice versa.
 * No matter how many Altars are destroyed, ore rarity remains set with Cobalt/Palladium being the most common, followed by Mythril/Orichalcum, then Adamantite/Titanium as the rarest.
 * Each time an Altar is destroyed, one or two 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.
 * Each time an Altar is destroyed, there is a 66% chance that a single random tile in the rock layer will be converted to Ebonstone/Crimstone or Pearlstone, facilitating the spread of underground Hallow or Corruption/Crimson. Only stone blocks can be converted in this way.
 * Additionally, smashing an Altar will start spawning mechanical bosses (until it's defeated, each has a 10% chance of spawning each night), and random Pirate Invasions (if all the criteria are met).

Hardmode Ore locations

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

Achievement
Begone, Evil! is the achievement you earn when you smash your first Demon Altar or Crimson Altar ( only).

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.
 * On the rare occasion that an altar spawns on top of a Heart Crystal, the bottom two portions of the crystal can be broken for two separate items, while the top two portions remain unbreakable.
 * Stacked Altars will bless the world with 2 ores

Trivia

 * The difficulty that new players often face in identifying and determining the nature of Demon Altars, often resulting in forum posts asking about "purple spiky things", has become a notable social phenomenon in the Terraria community. A title message references this with the text Terraria: What's that purple spiked thing?
 * In some rare cases an altar can spawn on another altar. Hammering it in Pre Hardmode will deal damage and cause both altars to break.
 * An Altar can spawn on sand, breaking the sand beneath it will break it,  ores will spawn only if it is Hardmode.
 * In some rare cases an altar can spawn on top of a crystal heart, causing the bottom half of the heart to be destroyed during the collection process, while the top half remains underneath the altar while also retaining its animation