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fusion/fission mechanic
you can now fuse 2 pokemon in-battle (if both are holding items then you get both effects). only a few can do this, like charizard x pikachu, venusaur x blastoise, milotic x gardevoir, etc...
their stats get increased by 1.5x everytime they defeat a mon
also, some old pokemon get a "fission" form, like gligar who is two pokemon fused into one
