Plantera

Plantera is a hardmode boss introduced in 1.2. It is summoned by destroying a pink plant that grows in the open areas of the Underground Jungle. The pink plant only spawns after at least one of the three "mechanical" hardmode bosses has been defeated. Upon death, Plantera drops a number of items, including a key that unlocks the underground jungle dungeon. When it is defeated for the first time it will always drop a Grenade Launcher with Rocket Is. Plantera changes to its second form when it reaches half health. In this second form, its mouth opens and charges around at the player, moving around somewhat quicker and erratically. Plantera will not despawn in any case apart from the player dying, or running far enough away from it (which is difficult to do in the Underground Jungle).

Trivia

 * "Plantera" is a parody of the American groove metal band Pantera. It was named as thus because Pantera is Redigit's girlfriend's favorite band.
 * Plantera also drops "The Axe", which looks similar to the Washburn and Dean electric guitars often used by the late Pantera guitarist Dimebag Darrell.
 * Plantera's boss music also consists of heavy usage of electric guitar, the basis of metal music.
 * 'Plantera' is the Swedish word for plant (as in "to plant").