Obsidian Skin Potion

Hellstone, mainly. Mining it dumps lava into the hole you just dug, and without this you spend the entire trip laying platforms and waiting for pools to drain. With it you strip veins at full speed and swim into the lava lakes for the ore sitting underneath, which is where a lot of it hides.

It also shuts off the burn from Meteorite and from the various Hellstone brick surfaces, so it doubles as the safe way to clear a fresh meteorite crash site before you own the accessory that handles it.

Half the recipe grows where you are going. The other half does not. Waterleaf only comes from sand and only blooms in the rain, so stocking up means either a desert trip on a wet day or a planter box garden at home. Set that garden up early if you expect to spend real time in the Underworld.

Dangersense stops flagging hot blocks while the buff is running, since they no longer threaten you. Do not read that quiet screen as proof the area is safe.