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.

Trivia

 * 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.