Magma Skull

Two things, and only one of them lasts. Fire debuff immunity is a quality of life fix for the Underworld and for Hellstone mining. The short lava immunity window is the part that changes how you move: you can swim a pool and climb out the far side instead of tunneling around it. The timer is brief and it does not refresh the instant you leave, so treat it as a crossing allowance and not a bath.

Its effect is identical to the Molten Charm, so wearing both only ever buys you the stacked timer, never a stronger version of the immunity.

Everything in the Lava Charm upgrade family adds its own window on top of the rest, and the demonic minecart adds another even though it sits outside that line. Fill most of your slots with them and lava becomes an ordinary liquid for long enough to strip a whole vein without surfacing, which is the one loadout where this earns a slot. Outside a dedicated mining kit, Terraspark Boots fold the same protection into something you were already wearing.