Shadow Chest

One Shadow Key opens every one of these in the world and is never spent, so the moment you find a key the whole Underworld is unlocked. Plan the trip around that: sweep every Ruined House in one long pass rather than heading back to look for a second key. They are scattered across the Underworld, usually sitting on or inside those houses, and there are more of them per world than there used to be.

Each one holds a single strong Underworld item alongside ordinary underground filler. That one slot is the reason to go, and the pool it rolls from covers most of what you would want out of pre-hardmode. Beyond the weapon, they carry potions you cannot brew yet, Lifeforce especially, which is worth saving for a boss attempt rather than burning on exploration.

In hardmode a locked one may not be a chest at all. The clean solution is to open every genuine chest before hardmode starts, because a Mimic can only ever wear the locked appearance. After that, any locked chest you come across is not one. Without doing that, a Lifeform Analyzer or Metal Detector is your only warning.

Gold Chests that generate very close to the bottom of the world pull from the same common pool, so an ordinary looking chest just above the Underworld is not stocked like the ones higher up.