Sacred Scrolls of the Order
These scrolls are a sacred relic, magically infused with the insights and teachings learned by great paladins of generations past as a physical manifestation of their oath. They are kept secret under lock and key, only to be used by anointed champions in the greatest moments of need.
A paladin can read this scroll to activate their level 20 transformation ability once (if the benefit is passive, the paladin gains that benefit until they finish a long rest). This renders the scroll non-magical, and the writing on them fades. The writing is unintelligible to non-paladins.
The scrolls can be created by a 20th-level paladin using a year of downtime and 25000gp of supplies.