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. Said plant only spawns after another hardmode boss has been defeated first. 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, which is 15000.

Trivia

 * "Plantera" is possibly a parody of the American groove metal band Pantera.
 * Plantera also drops "The Axe", which looks like an electric guitar.
 * 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").
 * It is possible that the name "Plantera" is a portmanteau of "Plant" and "Terror."
 * Plantera could possibly be a reference to the giant plant from Little Shop of Horrors

Strategy
There is an easier way to kill Plantera by luring it to the surface. Before you start the encounter build a rope on the surface up in the sky. Once you lured Plantera with you just climb inside the ropes and shoot down at the boss. Plantera won't be able to chase you anymore since it's tentacles require walls to hold onto. If you posses any flying capability just fly right above him at the surface while shooting down and dodging his projectiles.