Demon altars
They're used to craft boss summoning things
If you try to break it, it will damage you, unless you have the Pwnhammer from the Wall of Flesh, and if you smash it with that or a stronger hammer your world will be blessed with Cobalt/Palladium, Mythril/Orichalcum (for special anvil), and then Adamantite/Titanium (for special furnace)