Simple map taken from an imp statue. The Mirage RIddle can be found in the Altus Plateau. It's being held by an imp statue just outside a church not far from the Bower of Bounty site of grace.

Nonetheless, to start his questline you must reach the Altus Plateau. It's important to do ... which will take you to a giant statue with a message that says "Regressions alone reveals secrets".