In our research we have found a name of a Fae changeling that matches what that fallen druid had described. Cealhaidh the Corrupter is a faerie in the court of Twyl who has brought many mortal nobles to ruin.
Go to the Elven Palisades and touch this cold iron bar to the Aelfwar Elves, it will reveal the changeling. Bind him in a circle of ash before you destroy him and his spirit will have nowhere to go but into this crystal vial. Bring it back here to Sanctum and we will extract the truth.
Cealhaidh is not some minor faerie or flattering courtesan of Twyl's court. He is a powerful and crafty creature who has had all of creation to perfect his art. The gods have truly blessed you with skill and power for what you have accomplished. But now he is on holy ground, and once we find what he knows, there will be no escape.