[magic] spell to find members of a group of people?

[JoelSammallahti]

One of the players in my campaign about vampires wants a spell that can find the descendants of his tribe, who lived hundreds or thousands of years ago. We're using the standard magic system, but I'm not too familiar with it.

Is there a spell that can do this? Find members of a specific group of people, but not necessarily specific members of that group?

If there isn't, how would you write one up? Most importantly, what would be the prerequisites?

(Leaving aside the fact that due to basic population dynamics, pretty much everybody in their area is going to be a descendent of the tribe since it lived so long ago. I think I might rule that descent is strictly patrilineal, since that's the way the tribe in question reckoned kinship. That way members of the group remain just a subset of the total population, barring unusual events.)