I found this heart while exploring the crimson biome - what is it?

1 Answer

That is a Crimson Heart. You can smash it with a hammer to get an item. Every 3rd Crimson Heart that you smash will summon the Brain of Cthulhu boss, so make sure you're properly equipped before smashing your third one.

Each world can have several instances of the Crimson biome, depending on world size, and each chunk of Crimson biome will have several Crimson Hearts. In a large world you can expect in excess of 20-30 Crimson Hearts throughout.