The Arrival of SkullGreymon Digimon: Digital Monsters : Season 1 Episode 16

Joe's tag leads the DigiDestined to a coliseum, where Etemon traps them and forces Greymon to fight another Greymon. After Joe finds the Crest of Reliability, the group escapes and Tai forces Greymon to Digivolve to his ultimate form by putting himself in harm's way. Instead, Greymon Dark Digivolves into SkullGreymon, attacking everyone before eventually reverting to a regretful Koromon.