Prime source discovery - near Tascrig.
This discovery is a result of this chapter, specifically, the creation of Tascrig. Typically, the announcement would be made after the chapter is accepted, but I want to speed things up a little. You can describe this source in the chapter. Maybe they talk about a holy tree just outside the village, or an ancient tomb, or a magic spring, or really anything you want. The Prime source exists in the world, it costs nothing because you don't create it, you discover it.
Note that only councils can take control of Prime sources, so now might be a good time to form a council or two...