You were fighting Mitchell also known as Kamen Rider Surge.

During the fight you had knocked him out of his Rider form which seemed like it would be the end of the fight until he took out a different progrise key. This one being a ZetsumeRise Key

(not the ArsinoMagia key but the same color)
When he presses the activation button it announced "Megalodon" before he put it in his forceriser and pulled the switch opening the progrise key as it announces. "Forcerise. ........ Breakdown!" Transforming him into a new form.

When he was in his new form he said. "The day humanity becomes an endangered species is near." What will you do?