Illusionist's Bookmark

Wondrous item, very rare (requires attunement by a wizard)

This well-worn bookmark is cut from an old illusionist's scroll. While you're attuned to the bookmark, you can leave it wedged between the pages of your spellbook to cause its own magic to empower your illusions. When you use a spell slot to cast an illusion spell that you have prepared in your spellbook, other creatures take a –5 penalty to ability checks made to examine it or determine the nature of the effect. If the illusion spell is capable of dealing damage, you can also reroll any 1 or 2 on the damage dice. When you do, you must use the new roll. If you remove the bookmark from your spellbook, you no longer gain this benefit.

The grey wizard sat reading the collection of tomes and manuscripts with intrigue. While his knowledge of the arcane expanded, he made sure to mark a page with his trusty bookmark and write "Improvements needed" in the margin.