Book of Thothwondrous, legendary (requires attunement)

This leatherbound book of ancient parchment is sealed shut and you are unable to figure out how to open it, but just by holding it you can feel that there is untold knowledge within it. While attuned to this book you are able to speak with plants and animals as if affected by the spells Speak with Animals and Speak with Plants. Additionally, you have proficiency with all Wisdom checks and saving throws. As an action once per day you can cast either True Seeing (with a duration of 10 minutes), or Contact Other Plane, ignoring components.

Curse. If you do not cast either True Seeing or Contact Other Plane for at least one day while attuned to this book you begin to see and hear things. When you make a critical fail on an ability check, or at any time your DM chooses, you suffer one of the following effects: You see something that does not exist until the end of your next turn, something near you appears to change forms until the end of your next turn, or you hear the voice of another creature near you.

You become so used to seeing things that you shouldn’t, speaking with that that doesn’t speak, and encountering other planar beings simply by being near this book that you automatically fail the first saving throw for any illusory effect or spell from the school of Illusion because you assume that it is one of the many apparitions you often encounter.