Obsidifish

Obsidifish are a type of pre-Hardmode () / Hardmode () fish which are found very rarely by fishing in any biome in any layer, but only in lava. They are used to craft Seafood Dinners, Inferno Potions, and Potions of Return.

Crafting[]

Result	Ingredients	Crafting station
Seafood Dinner	Obsidifish2	Cooking Pot or Cauldron
Inferno Potion	Bottled Water Flarefin Koi Obsidifish2 Fireblossom	Placed Bottle or Alchemy Table
Potion of Return	Recall Potion Obsidifish	Placed Bottle or Alchemy Table

As Obsidifish are only caught in lava, the Lavaproof Fishing Hook, Hotline Fishing Hook, or any pole using Underworld Critters as bait can be used to catch them.

The fact that Obsidifish is required in the Potion of Return may be a reference to the Nether portals in Minecraft, as the fish itself is made out of obsidian, is found naturally relatively close to The Underworld, and is used in a potion to summon a portal between two areas.
Obsidifish have a strikingly similar resemblance to Obsidian Swordfish.