They can be used as either a Def or Off unit. Attack or defend however you please. They're att is 50 and def is 60. Players use them for blockades, in presidios or on Adventures in the Lighthouse generally. 

They can only be created in the cemetary by you clicking on friends or by friends going to your haven and clicking the floating skull. The max you can have is 50 at any given time. This can be increased though to a max of 100 using pearls in the Witch Doctors Hut

The second reason you may not see the floating skull is because you've been used as a dark corsair already and they haven't used that unit already. For instance if Fazi clicks my haven and I don't destroy that dark corsair in some offensive action. Then the next time Fazi visits my haven he won't see a floating skull. These are troops that are free no reason to store them. It limits your ability to get more once you reach the max. However you can increase the max using pearls in the witch doctors hut

Fairness