In "The Shadow in the Attic," a nephew inherits his eccentric uncle's gambrel-roofed house. Right off the bat, his uncle is characterized as a feared man. "Who could do more than whisper fearfully of what dark powers were at Uriah Garrison's command?" (For fun, insert Lord Valdemort in place of Uriah Garrison.)
A neighbor sees the coffin, yet doubts that it contains Garrison's body. The nephew retreats to his uncle's library, full of ancient and modern books on sorcery and witchcraft. It is these books, and the annotations made by his mysterious uncle, that lead the nephew to his understanding of his uncle's interest in succubi -- the retention of the "essence" from one existence to another.
This essence, or soul or life force, is very close to what any Harry Potter devotee would recognize as a horcrux: the essence of Lord Valdemort placed in various magically-protected items to be regained after his death. The nephew in this story deduces that perhaps his uncle was attempting to take over a new body by driving out the life force within and substituting his own essence.
Coincidental? Read on.
It is the short story "The Horror of the Middle Span" that provides the final straw that breaks the camel's plagiaristic back. The similarities heretofore were possibly, maybe, truly coincidental. The following, well, this is not coincidence.
In this story, a great-nephew travels to backcountry Massachusetts to lay claim to his great-uncle's abandoned house and property. In the course of his efforts, he finds the townspeople quite unfriendly. His great-uncle was not liked, but feared by the locals. In searching the house, the great-nephew comes upon some correspondence, with one letter in particular arresting his attention. The letter itself is of no interest here, but the letter writer's closing provides the coup de grace.
Only Harry Potter called Lord Valdemort by his self-chosen name. Everybody else quivered in fear, calling him "He Who Shall Not Be Named." In Lovecraft's story, the cryptic letter that the great-nephew finds so interesting concludes with, "I am yours in the Name of Him Who is Not To Be Named."
Artistic license or outright theft? You be the judge.
These examples come from just one small book of Lovecraft's short stories. What other "similarities" might be found if one examined his entire oeuvre?
Either way, should Rowling's fortune be taken from her and the books and movies recalled and destroyed? Absolutely not. She is a far better writer than Lovecraft and did wonders with the stones, horcruxes and characters' monikers.
But maybe a footnote should be added, giving readers the idea that Rowling didn't walk into Diagon Alley alone -- H.P. Lovecraft was a step or two ahead.