For wizard types. This is a diabolical item, originally meant to render a long-forgotten (and much despised) traveling wizard helpless.
This appears as a bottle full of the finest black ink. It will radiate magic. Investigation will reveal it to be superbly suited to transcribing spells into spell books; so much so, that any spell learned from such a spell book, can be cast as if the wizard were 1 level higher than they really are.
The curse comes in 3d10 days after the transcription has been finished. On the appointed day, the spell book will start smoldering and then burst into flames. This takes only a minute's time, so if the book is in a backpack while the wizard is out adventuring (which was the original plan, when the ink was first made), the wizard takes 1d6 HP of fire damage (no Saving Throw allowed) and they lose the book, along with anything else near it that might be flammable. Most especially, they lose their ability to learn new spells until they can recover them from another source, be it a second copy of their spell book, or from somewhere else.