This oily, crimson liquid is rare to find even in the more complete apothecary’s collections. It can be easily applied to any blade or arrowhead as it was designed to do so, however if consumed it will have no ill effects on the drinker.
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....)