You receive a vision of a rooftop "somewhere close by", where a rare moss grows; in the vision, you know the moss is suitable for greatly enhancing the effects of certain types of potions. Now go and find that rooftop. Don't fall off. (Of course, rooftop runners *will* be taken for miscreants by the local City Watch....)
Half hour off the Harrow Road you find it, Fool's Grave. Amid an overgrown garden sits a single tombstone. Carved neatly into the weathered slab is the word "Fool," with a symbol underneath matching the signet ring you found.