I dont quite see how bains post relates to the thread but I have to point out that Druids were Celtic practitioners and did not believe in or work with demons thus I find your story unlikely. As Cathage was also focussed on a Semitic faith I would be even more shocked to find of Druid evolvement.
On to the topic of necromancy then. Communing with the dead is not in itself inherently dangerous, people do it every day without even thinking about it by visiting their relatives graves and asking advice, of course they dont necessarily expect an answer however many people gain a sense of clarity from the experience. However as Muurta pointed out the more in depth ritual practices, as with any summoning or communing work, have greater potential to go wrong. With the correct research and preparation however these risks can be minimised and the practice can be conducted safely.
Except the attacking part and I heard someone (read in chat) that spirit may try to follow or possess if thy defense is not good enough,or actually well departed.I found nothing more dangerous than this. Other danger being that intruders may possibly arrive. Nacromanacy is more deeper than just connecting with a ghost. Yet usually many circles were casted before performing it and fasts were kept to keep body pure. Don't know how true is that fasting thing cause Nacromanacy is something I am not muchinterested in .