Armor Polish

Broken Armor halves your defense, and it comes from ordinary enemies rather than bosses, which is exactly what makes it dangerous. It lands mid fight while something else already has your attention, and everything that hits you afterwards lands for close to double. Cursed Skulls in the Dungeon are the classic case, since they phase through walls and surround you before you have anywhere to back into.

By itself it is not worth a Hardmode slot. Its real job is as one component of the Ankh Charm, and the shield that chain ends at is worth wearing for the rest of a run. Pick it up because you are collecting all of them, not because you plan to equip it alone.

The Dungeon after Plantera is the easy route, since the armored skeletons that carry it spawn thick enough there that one falls out without a dedicated farm. If you happen to have Vitamins instead, shimmer converts one into the other and skips the grind entirely.