He claims to be the rightful heir.

他要求做合法的继承人。

《新英汉大辞典》

He claimed to be the rightful heir.

他声称自己是合法继承人。

youdao

He claimed that he was the only rightful heir.

他声称自己是唯一合法的继承人。

youdao

But the sudden death of Kim Jong Il has accelerated the need for accounts of the younger Mr. Kim as the rightful heir.

WSJ: Pyongyang Turns to Task of Glorifying New Leader

People like to tag Kurzweil as the "rightful heir to Thomas Edison, " and that's not a stretch considering he's responsible for some of the most useful inventions of the past century.

CNN: STORY HIGHLIGHTS

Rodelinda says that before they can be married, he must kill her own son Flavio declaring that she can't marry Grimoaldo while the rightful heir to the throne is still alive.

NPR: Riches Rediscovered:? Handel's 'Rodelinda'