Sedna is their chief province, bordering the Novan duchy of Elath, and the nobility of both provinces have been known to intermarry (Foreign Affairs - 40).
Several generations back, the rulers of Elath and Sedna married. The citizens of Elath objected to the foreign influence, and the resulting heir died under suspicious circumstances, after which the duchy of Elath passed to a distant branch of the family rather than another child of the Elath-Sedna union (Internal Affairs - 70).
Talarist will present himself as a suitor for Elodie. Depending on how Elodie interacts with him, Talasse may become a steadfast ally of Nova or ruin the stability of their borders in hostile retaliation.
Even if Elodie doesn't marry Talarist, an alliance may be estabilished during one epilogue where she marries his brother Signuel instead.