Indeed it 'was' a Black Reaver....with a bit of a twist.
There is still a Vampire and an Elemental around (I added Elemental) and if the group manage to defeat all 4, they will make the Black Reaver inert. Not that they know that, just yet. In fact, they circumnavigated the part that hinted it was around, so they are actually managing to do something useful without gathering the intel. Even with 2 of the 4 'components' down, the Black Reaver has been reduced to the lesser varierty.
I think I must add that my campaign is fairly low on magical items, so these combats take place without the aid of the usual gear you may expect of such a level of party. I'm not sure whether or not that impacts on the RL time taken; less options, but also less power.