Erik is likely not based on a true story. Many such characters are invented to entertain or convey certain themes and messages, rather than being rooted in actual happenings.
In a different version, Christine gets lost in the opera house and Erik is the one who finds her. He could have been watching her from afar all this time. Instead of scaring her, he gently leads her to a safe place. They start talking about their dreams and fears. Christine realizes that Erik is not the monster everyone makes him out to be. This new understanding blossoms into love as they continue to meet secretly and share their innermost thoughts.
A simple fanfiction could be that Erik writes a beautiful song for Christine. When she hears it, she is deeply moved. She seeks him out to thank him. This encounter starts a series of meetings where they slowly fall in love. Their love is based on mutual respect for each other's talents.
It depends on the reader. Some people are more easily scared by ghost stories than others. If Erik Larson uses vivid descriptions of spooky events, like strange noises in the dark or ghostly apparitions, it could be very scary for those who are sensitive to such things. However, for more skeptical readers, they might not find it as terrifying.
One of his popular novels is 'The Devil in the White City'. It weaves together the story of the 1893 Chicago World's Fair and a serial killer who preyed on its visitors. Another is 'In the Garden of Beasts', which tells about an American diplomat in Nazi Germany.
Often, you'll find themes like redemption in Charles and Erik fanfic. Maybe Erik has to find his way back from a dark path, and Charles is there to guide him. Also, the theme of loyalty comes up a lot. Despite their differences, they have a deep loyalty to each other in many of these stories. And there's the theme of sacrifice too. They might be willing to sacrifice their own beliefs or even their safety for the other.