The dwarven catacombs are located in the forest beyond the stream north of the town of Vergen. (Use the stairs to the right of the town gate.) Within are a number of corpses and some breakable walls, not to mention a number of wraiths. As Geralt inspects the various corpses found there, wraiths appear, usually about three wraiths per corpse.
Ultimately, Geralt finds the wraith of Ekhart Hennessy there.