The Hound Whistle is needed to call the Hound of Doskar in Ultima VII Part Two.
The Forest Master Morghrim told the Avatar of it, but reminded the hero that the whistle would only work after returning the power orb from his foe Draygan, who had stolen it. The Avatar did so by killing Draygan with a special King's Saviour arrow, and received the whistle. The Hound was essential to track down Cantra and Batlin.