Cassius Fontayne I (b. May 2, 1994) was a part-vampire and the son of Mardon Fontayne VI and Erjona Hensul. Cassius was the first part-vampire to ever attend Hogwarts School of Witchcraft and Wizardry, due to the newly passed law in 2003 allowing him to attend in 2005. Cassius was sorted into Slytherin.
Cassius spent the first years of his life traveling the world with his parents. From the summer of 1995 to May 1998, Cassius and his little sister, Solaire traveled the world with their parents. They were too young to understand that it wasn't just a vacation. Their father Mardon Fontayne VI was on the run from Death Eaters who were sent to kill all half-blooded creatures such as part-vampires. Cassius and his family were unable to come home until after the second Wizarding War was won by Harry Potter, and they were once again safe.