I suppose without the website, that may become a more common question for new players.
He has a special attack called possess. When he is touching another figure, as an attack he may take control of that figure. You now control that figure. This is obviously the most fun on megas. And on normal sized figures he become very lopsided.
ok so I move the emperor and then as his attack he can grab another model. What happens then can he move and attack immediatly with the grabbed model? Does he control the model from a distance or does he have to stay grabbing it?