On the morning after every third summer equinox, in the middle of an ancient stone circle, the still smoldering embers of a fire can be harvested for 5 pawns of ignem vis. The embers will eventually cool down somewhat but will always remain warm to the touch.
If the characters go there the night of the summer equinox to try and discover what is going on, they can see a large bonfire fire at the spot from a distance. If they come closer to the stone circle the fire disappears and there is no sign of any activity. Regardless, the smoldering embers are always there in the morning.