Frozen 3 has been confirmed to be in Disney's future, and the unanswered questions from the conclusion of Frozen 2 imply that its plot may continue the conflict between Arendelle and Northuldra. The actions of Anna and Elsa's grandfather, King Runeard, were revealed in Frozen 2 to be at the center of a bloody battle between the two kingdoms, and the union of Iduna and Agnarr is the reason for Elsa's status as the fifth spirit of the elements. Still, these plots barely seemed to touch on the more profound history of Arendelle and Northulda, especially since the two sisters are now running each kingdom.

https://screenrant.com/frozen-3-arendelle-northuldra-anna-elsa-conflict/