Big Boo (Paper Mario series)

Atomic Boo is a Boo that is made of several Boos combining as one. The Atomic Boo is an optional mini-boss in Paper Mario: The Thousand-Year Door, where it is composed of approximately two hundred different Boos. To fight this boss, Mario must go to the Creepy Steeple after freeing the Boos in Chapter 4 and agreeing to be nice to them, after which all the Boos will come out and swarm him. If Mario knocks the Boos away twice with the Super Hammer by using the Spin Attack, the Boos will combine together to make the Atomic Boo. Atomic Boo attacks by flinging Boos at Mario, charging up a powerful glare attack, hiding his face and becoming invisible like regular Boos, floating over and falling onto both Mario and his partner, and inflicting Mario with status ailments such as confusion and paralysis. Once Atomic Boo is defeated, he will relinquish his Lucky Start badge to Mario and the Boos will stop their swarming and appear as normal enemies.

Atomic Boos are a rare enemy in Super Paper Mario. In fact, only one appears in Merlee's Mansion, where it is considerably weaker. This Atomic Boo will attempt to attack the player by charging at them while the are facing the direction opposite of them and will turn intangible and shy away if the player faces it. Also in Super Paper Mario, Atomic Boos will occasionally appear in the minigame Mansion Patrol; though slower than regular Boos, Atomic Boos take several hits to defeat instead a single one.

Trivia

 * Atomic Teresa, the Japanese name of the Atomic Boo, is also the name of the Big Boos. Therefore, Atomic Boos first appeared in Super Mario World, just under a different name outside of Japan.
 * A purple variant of Atomic Boo, called Dark Atomic Boo, was found in the game's coding, but cannot be encountered. It would have been fought on the 50th floor of the Pit of 100 Trials. It's reasonable to assume that Dark Atomic Boo would have used more powerful versions of Atomic Boo's attacks.
 * The idea of an Atomic Boo being made of many Boos may have come from Boolossus in Luigi's Mansion.
 * While Boolossus is made from fifteen Boos, the Atomic Boo from Paper Mario: The Thousand-Year Door is still smaller than Boolossus, even though he's made from two hundred Boos.
 * Boolossus and Atomic Boo have the same attack by trying to squish the player in Luigi's Mansion and   Paper Mario: The Thousand-Year Door, however the Atomic Boo in Super Paper Mario acted like a normal boo.