Can I cast Defy targeting a spell that costs more than 4 energy to get a spell cast trigger, either by targeting an invalid spell or by having Defy target itself?

Ruling: You cannot cast Defy targeting a spell that costs more than 4 energy, nor can you have Defy target itself. Defy requires a valid target with energy cost ≤4 and power cost ≤1 to be cast, and spells cannot target themselves.

Sequence:

• Defy must have a valid target to be cast (energy cost ≤4 and power cost ≤1)
• If no valid target exists, you cannot cast Defy at all
• Even if a valid target exists, Defy cannot target itself

Nuances:

• This differs from Magic: The Gathering where you can cast spells like Counterspell on uncounterable spells just to get cast triggers
• In Riftbound, Defy works like Red Elemental Blast (can only target valid spells), not Pyroblast (can target any spell but only counters if valid)