Abuse the interaction between copy mechanics and transform or undying to trigger morbid for free and build an army of demons.

Evil Twin copying Loyal Cathar is effectively immortal as long as a single Loyal Cathar remains to copy.

Evil Twin can kill an identically named creature for, triggering morbid.

Evil Twin copying Loyal Cathar can do this every turn.

Skirsdag High Priest can create a 5/5 demon after morbid has been satisfied.

While the top casting cost in the deck is 4 (Sorin and Evil Twin), until the combo gets started the deck is a little vulnerable. Avacynian Priest and Burden of Guilt will help lock down creatures while Forbidden Alchemy and Think Twice help search for your cards. Gravepurge is provided in case Forbidden Alchemy dumps a creature you need, or you find many creatures heading to the graveyard. Gather the Townsfolk serves three purposes: early blockers, morbid triggers, and creatures for Skirdag High Priest to tap.

Not using Evil Twin for Loyal Cathar or Skirdag High Priest is expected. That's why Undying Evil is in the deck. Like Loyal Cathar, an undying Twin will auto-revive and copy a new creature (with a +1/+1 counter). Try to chose Loyal Cathar for the new copy so another auto-revive will shake the +1/+1 counter. A copy of Loyal Cathar is a wonderful blocker. Undying Evil will also allow an Unhallowed Cather to return to being a Loyal Cathar.

Sorin is here primarily as a distraction. Play him and start dumping vampire tokens on the battlefield, which should buy a few turns. If you do get him to his ultimate, it can be used in a pinch to trigger morbid or rescue an Evil Twin that can't auto-revive. Or you can steal your opponent's stuff.

There is a distinct lack of counter spells. I don't care much for the counterspell play style, so this deck is not a control deck. It is a stalling deck.