Huge Dragon Egg

Every world places a small fixed number of these when it is created and never makes more. Small worlds get the fewest. There is no farming route: once you have cracked all of them, the only way to get another is a new world. Grab them the moment you spot one rather than marking the spot and losing track of it.

Spelunker highlights them and a Metal Detector calls them out, which is how you sweep for them while already mining the cavern layer. They never appear below the lava line, so running a hellevator past it hunting for one is wasted effort.

Opening one hands you one of the two Chillet forms and you do not choose which. With only a handful of eggs per world and a coin flip each time, collecting both usually eats most of what a world has to give. The egg and the way it hatches are a nod to Palworld, where the same item does the same thing.