Obsidian Skull

This stops the burn you take from standing on meteorite or hellstone. It does nothing about lava and nothing about anything that shoots fire at you. Walking into the underworld wearing it and expecting to survive a lava pool is a common and expensive mistake. For lava you still want the potion or boots built for the job.

The reason to make one is that it turns two ore fields from hazards into ordinary rock. A meteorite crash site is effectively unminable without it unless you are willing to eat damage on every swing, and hellstone is worse. With it on you work both at normal pace.

Equipping this stops Dangersense from highlighting meteorite and hellstone. That is correct behavior, since neither can hurt you anymore, but if you were using the highlight to locate ore you need another method. Spelunker is the buff you want for that.

It feeds a chain that ends at the Ankh Shield, and it is one of the few links in that chain you can pick up before hardmode. Magma Skull, Molten Charm and Obsidian Shield all descend from it too, so every path out of this item goes somewhere useful.