Missy is the Hybrid, a time lord who loves death and destruction, likes to betray his/her own side just to get what he/she wants

How did missy escape skaro in the witch's familiar?

A regeneration in progress as the Next Time trailer said...

Maybe it's the Master regenerating into Missy.

We always have the Master banging on about the timelords resurrecting him because he made the perfect warrior. Corrupted the Master since he was a child. He could be The Hybrid!