As of July 22nd, 2018 - Existence Games has ruled that when a player plays this card, they do not need a creature on their side of the field to activate the effect.
Example: Player 2 has three creatures in play. Player 1 has no creatures in play. Player 1 plays Dragonis Warden. Player 2 must select one creature to send to the bottom of their deck. Player 1 does nothing.
As of August 19th, 2018 - Existence Games has changed the ruling; this card requires that both players must have a creature in play that can be targeted by this effect.
This effect fizzles if either target leaves play prematurely.