It has grown hungry over the years, stalks its prey and chases it through to exhaustion before feeding on the memory of the hunt and the terror its victim felt. The prey does not remember the ordeal, only holding onto the memory that it was chased by some avatar of the things that lurked in shadow.
Someone created this horrific beast, and while no one will take credit for it the monster roams the forests and hidden places. When you eat human flesh, you develop a taste for it. For now, someone is keeping it happy. For now.
The Keeper of Secrets is a tool that the unscrupulous use for their own ends; since it is a spirit it does not differentiate between intentions. Should it find that it was ever used though, the offender would be greatly punished.
Mages? Are delicious.