Heir was the title held by the person to inherit a person's possessions or position. They were often the subject of assassination attempts.
In 2010, Prince Charles was the heir of the British throne, to succeed after the death of his mother, Elizabeth II. Also in this year, the Automatons came to Earth to search for Gavin, the heir to their planet after the death of Gavin's father. (TV: The Empty Planet)
Rascla was the self-proclaimed heir to the universe. He was defeated by the Fourth Doctor. (COMIC: The Body Snatcher)
Alexei Romanov was the heir to the Russian throne but was killed in a revolution before he could take office. (PROSE: The Clockwise Man)
Duke Giuliano of San Martino was nephew of Count Federico. He became the lawful heir to the duchy after his father died in 1492, but his accession was actively contested by his uncle. Indeed, he was the target of his uncle's murderous ambitions, and became the victim of Federico's numerous attempts on his life and reputation. (TV: The Masque of Mandragora)
Charlie Smith was the heir to the throne of Rhodia before his species and planet were destroyed by the Shadow Kin. (TV: For Tonight We Might Die et al.)
Princess Lixgaar was the nine-year-old heir to the Martian throne after the New Martian Republic executed the rest of the Martian royal family on New Mars. She was taken to Peladon and hid in the chamber of the Prisoner of Peladon as a temporary measure until she could be taken to Galactic Federation headquarters on Io. (AUDIO: The Prisoner of Peladon)
Archduke Franz Ferdinand was the heir to the Austrian throne. He was assassinated in Bosnia, which begun World War I. (PROSE: Human Nature)