There’s a dragon terrorizing Phandalin, and it’s been visiting cities and stealing their Great Magic Books from their libraries. Lucretia owns one or two Great Books, and even if it doesn’t know about BoB per se, a magical beast like that might be able to sense the presence of magical items like that on the moon. So she sends the Boys out to stop it before it comes after her Books. They track it down, fight their way into its lair (some magic beasties that like to hang around dragons, like cleaner fish on sharks), and finally see the dragon with its back turned, surrounded by open books. Taako is about to cast Fireball on it, when it senses them – “Sirs?!” It turns out that some errant adventurer came across Angus, thought that he might be a Mind Flayer because of his intelligence, and cast True Form (or w/e) on him. Transformed into his true silver dragon form, Angus panicked and flew away, but is too low level a mage to know how to undo the spell, so he’s been trying to research how to go back to his normal appearance.