From the Super Mario Wiki
Skellobits are enemies from Super Paper Mario that serve Bonechill in The Underwhere. They escape their prison along with him and under the orders of Count Bleck, invade the Overthere. Near the end of Chapter 7-4, the Nimbis and Skellobits have a huge battle. They also appear in Castle Bleck.
Their tattle implies that they are dead villains who were imprisoned to "suffer for eternity among the game-overed".
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