Evil Mewthree is the combining of Evil Mew and Evil Mewtwo. After using Magic Wizard's wand they fuse together and gain ultimate power. They becoming stronger than Arcetina and take over the world.

They have no weakness as not even good mewthree can defeat them. The evil is too strong.

He even has a special move called rein of curses where curses rain from the skies and affect anyone they hit.

Don't fight him. Just accept your fate. It's better that way.