A tome of dark magic.

During your time browsing the odds and ends of the curio stalls, you suddenly feel.... something...pulling you towards a pile of leather-bound tomes. Without understanding exactly what you're looking for, you put aside crumbling grimoire after moldy libram. Eventually, you uncover a hefty, black tome, secured by a clasp and adorned by ominous, vague suggestions of tentacles.

"Ah... I see you have an eye for hidden things." The creaky voice of the curio peddler makes you jump as his whispers almost in your ear.
"What is this book?"
"A tome of powerful magic. An adventurer such as you could surely benefit from such a purchase."

You have found the Dark Tome: You can study this book of black magic to attempt to learn useful magic, with the risk of corruption to your soul....and perhaps other risks as well.

"H-how much?"
"I suppose I can let it go for... 40 gold pieces."