From the Super Mario Wiki
Smorgs are small, black cloud-like enemies with yellow eyes. They appear in the sixth chapter, Excess Express in Paper Mario: The Thousand-Year Door, where they collectively form a large entity named Smorg, who serves as the chapter boss. Smorgs come in two variations: regular black versions and magenta variations. Magenta variations make up the central structure of the Smorg boss, while black ones consist the external parts of the boss.
Smorgs are first seen attacking the Excess Express and kidnapping its passengers after being released in the Riverside Station. Mario saves the conductor and the waitress from the Smorgs, which makes them realize that everybody must have been attacked in their sleep. Smorgs cover the roof of the train, where Mario makes his way towards at the front of the train, where they form Smorg. After Mario defeats Smorg, the Smorgs blow away and release the captive passengers.
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