Lets say you have 1 cat, and your opponent has 5, you use this ability and then do all the other cards die, and this one stay live?
I demand you to screenshot you having 1 cat and your opponent having 5. Now.
...Otherwise, I assume it affects both fields.
Too lazy to screenshot, but I'll give you an easy card to realize it: Parallel Universe + This.
Or if you are thinking of a possible openEtG port... Beguile + This.
Need a relevant situation where you would bother?
Your opponent has several armagios. All of them have overdrive except one which is on shield duty and at low HP.
You copy the highest ATK armagio with PU, then pop the shielder with this so that all of the enemy armagio die.
@Pineapple, I see your point here on the power cap issue.
However, this is really only different from a standard mass CC spell in one respect (albeit an important one).
This card's effective damage output per creature affected is not fixed whereas other mass CC cards are.
I think balancing that could be easily done by adding in a required cost absorb based on total damage dealt.
E.g. "Deal 4 damage to target. Kill all copies if it dies from this. Absorb X
where X is 1/4 the net HP of the victims."
