Unlike the other dragons, the name of the Black Dragon was bestowed by the Moon. Everyone knew that his father's name was never known to anyone, not even him the son or the King of the Dragons. His name was especially a sensitive subject as the said name could bound the Black Dragon's soul to the person who had learned the name. The reason why his name was so important was also due to the fact that the black dragon is extremely powerful. His power could become a double edged sword that would hurt the people around him, thus it's been decided that the one who knew the name would be the one to stop the power. But this person has to be someone extremely trustworthy, someone who would use this to take control of the Black Dragon.
Magnus never knew whether his mother knew his father's real name. He suspected that she doesn't, after all, they barely had time to each other.