The Ebon Glass

From publisher blurb: Recently we have come under attack by foreign raiders. No one knows where they come from, they appear in the night, take prisoners, and disappear just as mysteriously as they came, not a trace of them found. A small tower on the border has been silent for over a month. The lord of the tower was a friend of mine. I am in need of brave souls to discover what has happened. Will you help us?