Aatrox, The Darkin Blade

When Aatrox, The Darkin Blade enters the battlefield, sacrifice target creature you control, Aatrox then gains power equal to that creatures power, Aatrox can transform into Aatrox, The World Ender, if you control The Darkin Blade and meld for {4}. He then Becomes a 7/5 with lifelink.