That would be the Nightmare Pickaxe. You'll need Shadow Scales and Demonite Ore in order to craft it. Kill Eyes of Cthulhu for the ore (one is enough for a pickaxe, usually) and Eaters of Worlds for the Shadow Scales. You're gonna need 12 Demonite Ore and 6 Shadow Scales. When you are ready with your resources, simply craft the pickaxe at an Anvil.
