Digimon: Digital Monsters - Season 1 - Eps 16: The Arrival of SkullGreymon
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.
About Digimon: Digital Monsters

Title: Digimon: Digital Monsters
First Air Date: 1999-03-07
Last Air Date: 2001-03-25
Status: Ended
Rating: 8.1/10 (from 845 votes)
Language: JA
Seasons: 2
Total Episodes: 104
Network: Fuji TV
Genres: Animation, Action & Adventure, Sci-Fi & Fantasy, Kids
Production Companies: Toei Animation, Toei Company
Synopsis
While at summer camp, seven kids are transported to a strange digital world. In this new world they make friends with creatures that call themselves Digimon who were born to defend their world from various evil forces.
Cast

Reiko Kiuchi
Daisuke Motomiya (voice)

Megumi Urawa
Iori Hida / Armadimon (voice)

Rio Natsuki
Miyako Inoue (voice)

Hiroaki Hirata
Takeru Takaishi (voice)

Romi Park
Ken Ichijouji / Digimon Kaiser (voice)

Junko Noda
V-mon (voice)

Kae Araki
Hikari Yagami (voice)

Koichi Tochika
Hawkmon (voice)

Miwa Matsumoto
Patamon (voice)

Naozumi Takahashi
Wormmon (voice)