Destroy "Archfiend Cavalry" by using "Oops!" while "Archfiend Emperor, the First Lord of Horror" is in the Graveyard.

You can Special Summon "Archfiend Emperor, the First Lord of Horror" with the effect of "Archfiend Cavalry."

In addition, you'll be able to activate the effect of "Archfiend Emperor, the First Lord of Horror," since there is an Archfiend monster in your Graveyard.