If I use an attack that Knocks Out a defending Gengar with the "Castastrophe" Poke-BODY, but it also knocks itself out, does the attacking Pokemon go to the Lost Zone, or is "Catastrophe" no longer in effect when Gengar is KO'd?

Since both Pokemon are KO'd at the same time, Gengar's Catastrophe Poke-BODY is still working at the point when Knock Outs are evaluated. So the attacking Pokemon would go to the Lost Zone.

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