Altars

Altars cannot be freed, picked up, or crafted.^[1] Demon Altars and Crimson Altars are crafting stations that generate naturally, typically located within and near chasms in The Corruption or Crimson, with rarer appearances underground.^[1] Pre-Hardmode boss-summoning items, the Night's Edge, the Void Bag, and the Vault are all crafted at these stations.^[1] A small amount of light is given off by placed altars, which aids in locating them.^[1]

When a world lacks any altars, the Eye of Cthulhu will drop a Demon or Crimson Altar item.^[1] Altars become destructible in Hardmode using any hammer with a hammer power of 80% or more.^[1] Striking an altar with a hammer that does not meet the power requirement causes the player to lose half their health, while the altar remains intact.^[1] Altars have a use time of 15 (Very fast) and require 1 for research.^[3]

Quick Facts

Type: Crafting station Background object
Dimensions: 3 wide × 2 high
Use time: 15 (Very fast)
Research: 1 required

Facts from the infobox of Altars on the Terraria Wiki (wiki.gg, CC BY-NC-SA 4.0).

Hardmode

Every three altars destroyed counts as one round, and the ore types generated are Cobalt or Palladium, Mythril or Orichalcum, and Adamantite or Titanium in order.^[2] Breaking an altar also permits natural spawning of mechanical bosses and Pirate Invasions when their respective conditions are fulfilled.^[2]

References

"Altars" (opening section) - Terraria Wiki on wiki.gg, revision 1033094 terraria.wiki.gg
"Altars", section "Hardmode" - Terraria Wiki on wiki.gg, revision 1033094 terraria.wiki.gg
"Altars" (infobox) - Terraria Wiki on wiki.gg, revision 1033094 terraria.wiki.gg

Text adapted from the Terraria Wiki on wiki.gg (CC BY-NC-SA 4.0), used by Freedom Games LLC, the operator of wiki.gg and Codexery. Page: Altars (contributors). Codexery summarised the page: each numbered sentence above was checked against the wiki section it cites before publication, and this entry is shared under the same licence.