Digimon Adventure 02, episode 13, "Dagomon no Yobigoe"
, written by Chiaki Konaka
In the english dub, Digimon, episode 67, "His Master's Voice", written by Jeff Nimoy
The Japanese name of this episode could be translated as "The Call of Dagomon," which makes the relationship to the Cthulhu Mythos a tad clearer.
Hikari (voice of Araki Kae) is drawn into an alternate dimension, known as the Dark Ocean, by the Hangyomon (english Divermon), who served Dagomon (english Dragomon, which ruins the reference to Dagon), but have fallen prey to the Digimon Emperor's dark spirals.