A Stolen Song

From the introduction:

The astronomer monks of Farsky failed for decades to tell the future by the stars. Desperate, they found a promising seer, and passed off her predictions as their own…until deadly illness struck. Unable to give up their lucrative scam, they trapped her spirit, which became a thing of evil: a banshee, whose howls foretold their doom. The party have heard rumors of a ruin, where once sages could answer any question...