There are two books with this exact same plot.
In one, Fergie goes and swipes a book from the library, unwittingly putting himself under the spell of an ancient evil warlock who pushes him to use the book's diabolical power to hurt those who wronged him. In the other, Rose Rita steals a scroll from a museum, unwittingly putting herself under the spell of an ancient evil witch who pushes her to use the scroll's diabolical power to hurt those who wronged her. In each case, the spellbinders in question entomb the ensnared protagonist in the same place that they themselves were buried long ago. The stories read the same, as if the author created a template and merely filled in different names and applicable pronouns, which is kind of a gyp since I had to pay for both books.
Jarmyn Thanatos was the only thing that really stood out about this book. A master spellbinder who hob-nobbed people like Cogliostrio and John Dee, he has a lot of tricks up his sleeve and he didn't make it easy for Professor Childermass, Johnny, Father Higgins, or Sarah.
One of my favorite scenes was when he conjured an army of insects to guard his fortress of doom. Seems pretty bland, right? But it turns out these insects were the mummified remains of billions of dead flies that he swoop down on Johnny and the Prof, filling their noses and mouths with their decaying corpses. That was just nasty right there.
That was one of the more inspired tactics of a villain who actually thinks that rearranging his rather peculiar name (Jarmyn Thanatos into Thomas Jannatry) would totally deceive a team of experts like Professor Childermass and Dr. Coote. He was kind of clever at times with the names but eventually you wonder how he got away with that kind of garbage throughout the years.
You have to feel sorry for Jarmyn Thanatos though. He's the villain of a book that's a retread of at least one other by the same author, he has a pretty lame motivation ("I want to live forever!" "Uh, why?") and his last name was THANATOS.