Can you play Harnessed Dragon if the opponent doesn't have a unit out?

Ruling: Yes, you can play Harnessed Dragon even if the opponent has no units. The ability will fail to finalize (enter the chain) if there is no valid target.

Sequence:

• Play Harnessed Dragon and it resolves fully
• The "When you play me" trigger is evaluated
• If no enemy unit exists, the ability fails to finalize and does not enter the chain
• There will be no chain item to react to or resolve

Nuances:

• This is not a Silver Rule situation about fulfilling as much as you can during resolution
• "When you play me" triggers are evaluated after the unit fully resolves
• If killing a unit were required to play Harnessed Dragon, it would need to state "as an additional cost"