Nightmare Pickaxe

This is the tool that opens hell. Before it you can stand on a lake of Hellstone and not touch a block of it, and every piece of molten gear sits behind that one restriction. On a first run it is the main reason to go and fight the Eater of Worlds at all.

The Deathbringer Pickaxe does the same job a fraction quicker. The gap disappears in practice, because underworld mining is spent managing lava, not swinging. If your world rolled corruption this is simply your copy and there is nothing on the other side worth crossing for.

Its own replacement, the Molten Pickaxe, is made from the ore it just unlocked. Take enough water or obsidian skin down with you to come back with the armor and the upgrade in one visit instead of three.