What rassendyll promise to do after tour of ruritania with his sister-in-law?

After his tour of Ruritania with his sister-in-law, Rassendyll promises to return to Ruritania to help the rightful heir, Rudolf Elphberg, reclaim his throne. He is motivated by his sense of duty and the strong bond he has developed with the people and the royal family during his visit. Rassendyll's commitment reflects his growing sense of responsibility and honor towards the Ruritanian cause.