Altars

Demon Altars and Crimson Altars are naturally-occurring crafting stations found mainly in and around chasms in The Corruption or The Crimson, and less frequently underground. They are used to craft pre-Hardmode boss-summoning items and the Night's Edge, as well as the and. 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 Hardmode, or with a hammer with less than 80% hammer power (in Hardmode) causes the player to lose half their health and will leave the altar unaffected.

Hardmode
After the Wall of Flesh has been defeated in a world, altars can be destroyed with any tool that has a hammer power of at least 80% (typically the Pwnhammer). Each destroyed altar will spawn a Wraith and causes one type of Hardmode ore to appear in the world, with the appropriate status message displaying in the chat: "" 
 * 1) First altar:  or, chosen randomly.
 * 2) Second altar:  or, chosen randomly.
 * 3) Third altar:  or, chosen randomly.

Players will be able to find these new ores scattered across the Underground layer to near-Underworld levels, with them replacing most blocks created during world generation. They will spawn at random locations in varying quantity, in places explored and unexplored alike.


 * Destroying more altars will never spawn Hardmode ore types beyond the three that were randomly assigned to the world during the initial round of destroyed altars. A world with Palladium Ore deposits will never spawn Cobalt Ore deposits, or vice versa (with the exception of ).
 * Which ores are generated is not determined until the first, second, or third 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/Orichalcum with the second altar, and Adamantite/Titanium for the third.
 * Each time an altar is destroyed, there is a chance that a single random Stone Block in the Cavern layer will be converted to Ebonstone/Crimstone or Pearlstone, facilitating the spread of Underground Hallow or Corruption/Crimson. No block other than the selected one will be converted initially.
 * Subsequent altars destroyed will begin the cycle again, though the fourth to sixth altars will spawn half as much ore as the first ones, the seventh to ninth will spawn only one third of the ore of the first ones, and so forth. So, the nth cycle will only generate one-nth as much ore as the first one. For example, destroying the 19th altar would yield one-seventh of Cobalt/Palladium Ore compared to the first altar. Destroying 12 altars will roughly double the ore provided by the first three; tripling that original allotment would require destroying 33 altars. Notice that although each subsequent destroyed altar will yield less ore, the total amount is (except by the number of altars in a world).
 * 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.
 * Additionally, smashing an altar will allow mechanical bosses to spawn naturally (if said boss has not been defeated before it will spawn with a chance each night), and random Pirate Invasions (if all the criteria are met).

Tips

 * Altars emit a faint glow, making them somewhat easier to find in unlit areas.
 * It is not uncommon to find an altar relatively close to the world spawn point. Wherever the player has set up their base, it is recommended to begin searching near 0 depth, and upon finding an altar, build a room around it so the player can craft in relative peace.
 * Players about to enter Hardmode may benefit by mining paths to several known altars beforehand, as navigating Hardmode Corruption/Crimson is dangerous at first.
 * Altars are needed for crafting certain items, hence destroying every altar in a world is not recommended.
 * It is not strictly necessary to smash an altar; all ores obtained by doing so can also be obtained by fishing up crates, allowing access to all ores and their counterparts in one world.
 * When destroying multiple altars in rapid succession, the resulting Wraith swarm may become difficult to deal with. Therefore, it is not recommended to smash another if the Wraiths are not defeated.

Trivia

 * The difficulty that new players often face in identifying and determining the nature of Demon Altars has become a notable social phenomenon in the Terraria community, often resulting in forum posts asking about "purple spiky things". A references this with the text ""
 * In some rare cases an altar can spawn atop another altar. Hammering it in pre-Hardmode will deal damage and cause both altars to break.
 * Destroying the lower one in Hardmode will destroy both, with the full effects of destroying two regular altars.
 * In some rare cases an altar can spawn on top of a Life Crystal. Both lower quarters of the heart can be collected as two separate items, but the top half stays unbroken under the altar. The remaining half still exhibits its normal animation after the bottom half is broken.