Montangue the younger, Montangue the elder (his sibling) had been killed in one of the many assassinations attempts on his father.
Ironically, Montangue was 58 years old when he was crowned. According to surviving records, his was an unremarkable (and brief) reign. His most significant achievement was to come after his death. His daughter Ostel (his only child) was hail and in her prime when Montangue died of fever.