King Boo and his underlings took shelter within the mansion‘s walls. During this time, King Boo decided to make use of his powerful army of ghosts to exact revenge on the Mario Bros. as pay-back for all the Boos that fell to the brothers’ hands in their past adventures. He does this by inviting them both to the mansion under the premise that Luigi had won the mansion in a contest. Mario arrived first, and King Boo made swift work of him, trapping him in a painting and stowing said painting away in the Secret Altar beneath the mansion.
