Forgotten

Most wizards agree that the Armageddon Spell is the rarest spell.
As the High Mage of The Byzantium Library, I know that it isn’t.
The rarest spell is The Lost Spell Of Forgetting, of which the only copy is in the Library.
Why is it The Lost Spell?
Because I lost it.
I have no idea where the spell is in all these shelves and cupboards and desks.
I’m surprised I even remember there’s a Forgetting Spell.
Just reading it makes you forget what it is.
Hey… that’s strange… what’s this in my pocket…
It’s a scroll.
Of… um… what?