The connection there is between Light's Heart (the sentient core of Xe'ra) and the fact that only someone in Xe'ra's lineage can open the core to access its memories. We take the core to Velen, who lets us know that O'ros is in Xe'ra's line and can help us open Light's Heart. But then O'ros gets ganked by the Legion, and we're basically left with an unlockable Naaru core. Khadgar finds some anecdotal evidence that Elune might have created the Naaru, and so we use the Tear of Elune to try to unlock the core and it works - which seems to cement the fact that Elune is somehow related to the Naaru, enough that Xe'ra's core recognizes her energies as "in the lineage of Xe'ra" and thus works to open the core and let us view its memories.