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 haunting the halls. A wraith may also appear to defend any corpse that Geralt decides to disturb, making investigation treacherous.
Ultimately, Geralt finds the wraith of Ekhart Henessy there.