satellite
Go through the alley at the north-eastern edge of Svartrau City, below the city walls.
Use Focus Mode to scan for outlined objects and inspect them.
Follow them across the roofs to the room with the vampire boss.
Win the fight; Song of the Mountain starts on its own afterwards.
Only what a source states. A quest with nothing listed is a quest whose rewards are unpublished, not one that gives nothing.
The two sources give incompatible starts: an alley found by exploration, or a note read on Ambrus's body after his duel. The alley is recorded because that account describes the whole chain step by step; the other is a one-line table row.