From Dark Magic to Dark matter.

This is based around an idea of an alternate version of "The Beginning of the End" because you cannot kill of a villain of such power that easily. the idea here is that the power of harmony/friendship only destroys his pony form and a second form appears, free of the constraints of a physical form and is able to use his power at it's full potential.

My choice would be a form that resembles Dark Matter and Zero from the Kirby games, because there's something menacing about a giant spherical mass with a single eye at it's center.

He'd then start amassingt his full power by absorbing all the mind controlled ponies and then proceeds to cover equestria in darkness while possessing as many lifeforms as he could. At this stage he's less a pony and more like a anthropomorphic black hole.

The "boss fight" with him would be hard and would take even more effort to defat him. using the power of friendship at it's full power as well as the magic of the alicorn princesses being combined in ordr to defeat this second form for good.