Further, he demanded that Prince Djem--the brother of Sultan Bajazet, who was in the Pope's hands--should be delivered up to him as a further hostage.
This Djem (Gem, or Zizim, as his name is variously spelled) was the second son of Mahomet II, whose throne he had disputed with his brother Bajazet on their father's death.

He had raised an army to enforce his claim, and had not lacked for partisans; but he was defeated and put to flight by his brother.

For safety he had delivered himself up to the Knights of Rhodes, whom he knew to be Bajazet's implacable enemies.
They made him very welcome, for d'Aubusson, the Grand Master of Rhodes, realized that the possession of the prince's person was a very fortunate circumstance for Christianity, since by means of such a hostage the Turk could be kept in submission.

Accordingly d'Aubusson had sent him to France, and wrote: "While Djem lives, and is in our hands, Bajazet will never dare to make war upon Christians, who will thus enjoy great peace.
Thus is it salutary that Djem should remain in our power." And in France Djem had been well received and treated with every consideration due to a person of his princely rank.
But he appears to have become a subject of contention among the Powers, several of which urged that he could be of greater service to Christianity in their hands than in those of France.

Thus, the King of Hungary had demanded him because, being a neighbour of Bajazet's, he was constantly in apprehension of Turkish raids.