Activate the effect of "Dark World Lightning" to send "Ceruli, Guru of Dark World" to your Graveyard.

Activate the effect of "Ceruli, Guru of Dark World" from your Graveyard to Special Summon it to your opponent's side of the field.

Furthermore, since the effect of the "Ceruli, Guru of Dark World" Special Summoned to your opponent's field will activate you can discard "Reign-Beaux, Overlord of Dark World" from your hand.

This will count as "Reign-Beaux, Overlord of Dark World" being discarded by your opponent's card effect, allowing you to use its powerful effect.