Obsidian Crate

These only come out of lava, and fishing lava before Hardmode is circular. The Lavaproof Fishing Hook that lets you do it normally comes out of these crates, and the Hotline Fishing Hook is a Hardmode reward from the Angler. The way in is bait: Hell Butterflies, Lavaflies and Magma Snails all work, and catching any of them means finishing enough Angler quests to own the Golden Bug Net first. Expect a long run of errands before your line ever touches lava.

The payoff for that work is Underworld metal without swinging a pickaxe at it, in ore and bar form both, plus an outside chance at a Hellforge so you do not have to tear one out of a ruined house. A patient player can be wearing Molten armor before killing anything, which inverts the usual order of a run. It is slow and it only makes sense if you already enjoy fishing, but nothing else offers that shortcut.

One roll covers Lava Charm, Flame Waker Boots, the Lavaproof Fishing Hook and the rest of that line, so a crate gives you at most one of them. The Hook is the entry you want first, because it ends the bait dependency and lets you fish lava properly from then on, or upgrade into the Lavaproof Tackle Bag. The Lava Charm is the one that makes the rest of the Underworld survivable while you are down there.

Every crate contains an Obsidian Lock Box, which is the part that stays relevant deep into Hardmode. Shadow Chests never regenerate, so once you have picked the Underworld clean, these and Hellstone Crate are the only remaining route into that loot table. An Ornate Shadow Key turns up occasionally on top of that.

Most crates also hand over an Underworld painting and one of the potted hellish plants, and a Demonic Hellcart shows up now and then. None of it affects a run. If you are building down there it is free decoration you would otherwise never see, and if you are not, it is stack filler.