Pretty Polly by Barbara Hambly

A novelette of 10,000 words, sequel to Hambly's Darwath series. The Keep of Dare is an enclosed fortress: no one knows all its secrets and corners. When people start disappearing, the Archmage Ingold Inglorion and his two Californian companions - warrior Gil Patterson and novice wizard Rudy Solis - are faced with the possibility that some deadly entity is hiding in the darkness of the Keep. And the situation is not improved when Ingold himself disappears.