Banshee (animal)
Banshees are cruel spirits bent on dominating both living and ethereal beings. They are noted for their Magical potency and exceptional beauty, often forming small "cults" of enthralled creatures around themselves if allowed to operate unchecked. Their mortal origins are fairly murky and disputed in academic circles.
For all the superstitious peasants erroneously branding a local ghost a "banshee", the frustrating reality is that a genuine banshee is incredibly unlikely to leave behind direct witnesses of its activities. An encounter with one of these spirits is certain to end with the victim dead in the best of cases, enthralled in the worst, kept imprisoned as a plaything if fate chooses to be especially cruel.
Locating the lair of this vile spirit was, therefore, a rather impressive feat of research and magical tracking in and of itself. But for all the difficulty of this step, the next is going to be far worse - not only will we have to face the banshee and its powerful Magic, but the numerous mortal and ethereal servants it has bound to itself over the eons.