You can begin this quest after defeating Medusa.
 See the fake Shepherd at Pendrago (Shepherd Malfore).
Head to Gododdin and you will see purple near the school. Go there and speak again with Malfore. You should see a new purple star appear. Go there and talk to a kid.
Now head to Yder Cave. Go to the spot where you can fight Dirt Leech (refer to THIS PAGE for its location and strategy). Defeat it and you will get an Yellow Iris Gem: Tri. Go back to Gododdin and speak with Malfore again (marked by “!”). The blessing for Gododdin will be restored :).