Hello,
Is it possible to play Tzeentch's Firestorm with only one valid target?
Hello,
Is it possible to play Tzeentch's Firestorm with only one valid target?
No, you must have 2 valid targets when you play the spell. If only 1 valid target remains when it resolves, it still does 2 damage to the remaining target though.
I think it will be the same thing with "plague bomb" in the next BP?
Correct. You can't play it without 3 targets.