Ancient leather bound spell book. Intricate carvings on the book are worn away by years of use. The pages are full of arcane passages and meaninless writings. Scattered throughout the book are unique spells that are very simple to learn. Ther rest of the book is totally unreadable by any means.
The book was created by Raflond the mage. He was of a selfish nature and therefore a target of many heists. So he created a few ‘special’ magic books to allow the few that got through his defenses to think they accomplished something. Little did they know of the nature of the book and the effects it had on those that used it.
Every spell in the book works, except not how it ‘should’ work. Put whatever spells you like in there and either let them work but give them a side effect, or totally twist the spell around in some way. The book isn’t ‘cursed.’ The spells work exactly how the mage wants them to.
There are many possibilities for effects. The famous magic missile could actually miss would be a good one. I like the subtle ones like their spell modifiers or other bonuses could go down without them knowing it. “What do you mean I missed, I rolled a 19.” That sort of thing. Something that would take a little bit to figure out, and once they figure out something is going on, what is causing it?