Use the Enormous Bird Call at one of Blaithe's Nests perched atop the Rim of the World. Kill Blaithe and gather one of his Ancient Feathers.
A winged visage? Could it be...?
<name>, there's no time to waste. You may have been speaking to Aviana herself, trying to return to us.
I'll need something from Aviana's past to reach out to her. High atop the ridge overlooking this shrine, you'll find a series of huge nests belonging to Blaithe, an ancient bird of prey that was once her consort. For ten thousand years he's been mad with grief.
Go to one of his roosts, and use this call to summon him. Kill Blaithe and bring me one of his feathers.