Vibrant Reverie/Rancid Rafflesia

Rancid Rafflesia is a late Pre-Hardmode. It's a huge poisonous bulb which is able to move in the jungle at will, being able to produce his own flies and breath gas within a wide range, making it a very dangerous jungle denizen.

Spawn
It can only be spawned by crafting and using it in Underground Jungle.

Behavior
Unlike all other bosses, Ivy Plant is unique in that it's stationary. While this makes it very vulnerable to attacks, it regularly produce No Good Fly, which themselves are very fast but vertically slow. Moreover, Ivy Plant will then blow a Poisonous Gas, having a very large and wide area of effect, and also poisons the player. However, the damage and speed of the gas reduces very fast, which makes it very easy to dodge the further the player is. After blowing the gas, it then becomes temporairly invincible while relocating itself nearer to the player, by shrinking and reappearing under the player's feet. After relocating, it repeats it's behavior.

When the boss follows you outside of the jungle, it produces a massive amount of flies.

Trivia

 * This boss was previously known as the “Ivy Plant”.