Enter the Dark Masters Digimon: Digital Monsters : Season 1 Episode 40

As the DigiDestined return to the Digital World, they encounter Chuumon, who tells them that the Dark Masters have taken over the Digital World while they were in Tokyo. The DigiDestined face the Dark Masters, who overpower their Digimon, and Chuumon sacrifices himself to protect Mimi. Pixiemon arrives to save the DigiDestined, sacrificing himself to allow them to escape.