Atomic King Boo

Atomic King Boo is an enlarged version of King Boo and the final boss of Luigi's Mansion 2: Revenge of King Boo.

Before the Transformation
Before King Boo became Atomic King Boo, he gathered 150 Boos from the Boo Woods and powered them up. He also ordered 25 of them to fuse in order to become Atomic Boo and 50 of them to fuse in order to create Boolossus. When King Boo was done ordering his new minions around, he sent a letter to the Mario Bros. saying that E. Gadd invented a new gadget that would make things easier for them. As soon as the Mario Bros arrived, King Boo and Boolossus appeared before them holding E. Gadd and the Poltergust 3000. As soon as King Boo finished gloating, he ordered Boolossus to capture Mario. When King Boo returned to the Secret Altar, he immediately opened a portal to the Spirit Dimension and tossed Mario in it. When Mario arrived in the Spirit Dimension, Lord Boorus, King Boo's father, captured Mario.

Area 4
When Luigi made it to Area 4, King Boo had already absorbed the other 75 Boos that were left in the Mansion. He then caused a wicked form of Bowser to take Mario's place.

Area 8
When Luigi made it to Area 8, King Boo had sucessfully absorbed the last of the 150 Boos. He then sucked Luigi into the Spirit Dimension and began a battle with him. Yet even with new powers, King Boo was soundly defeated by Luigi.

Trivia
The way King Boo became Atomic King Boo was similar in the way more than a thousand Boos fused to become Atomic Boo in Paper Mario: The Thousand Year Door.

Even though Atomic Boo from Paper Mario: The Thousand Year Door was composed of more than a thousand Boos, Atomic King Boo was still 2 times the size of it. This could be due to the fact that King Boo Super-Powered the 150 Boos he merged with.