Rathyrian Black

From the introduction: The characters arrive in Tannerbury Bridge, a small village. That night, they are attacked by invisible assailants who cry out in a strange and archaic language. Upon seeking more information, they learn about the Tower of Kohn Rathyr. Investigating the strange tower leads to conflict with more invisible assailants, several rooms full of strange occurrences, a dragon, and the emperor of old, Kohn Rathyr himself.