"Madness was in all the Jermyns, and people were glad there were not many of them."
This family and their history is the basis of the story "Facts concerning the Late Arthur Jermyn and His Family".
Little is mentioned of the family's history prior to the birth of Sir Wade Jermyn. However, it's said they were respectable enough and many had portraits, suggesting they have a long history of wealth. Sir Wade Jermyn was a famed anthropologist and explorer who was among the first to explore the Congo region. He kept an extensive collection of unusual artifacts and specimens he acquired in his travels. During one of Wade's expeditions to the Congo he found a lost city controlled by a race of highly evolved white apes. For unknown reasons these creatures make Wade their king. He took one of their females as his bride and after fathering a child with her they returned to Britain. Although Wade kept her hidden from his staff and the outside world, claiming she is an extremely shy Portuguese girl. He gave her a locket with the Jermyn coat of arms on it. In 1765 he was committed to an insane asylum in Huntingdon.
Sir Arthur Jermyn was an English nobleman and the final descendant of Wade Jermyn. Lovecraft described him as a poet and a scholar who had come to accept his appearance. When learning of his ancestry he soaked himself in oil and set himself on fire, putting an end to the Jermyn family.