The true patronus was discovered (possibly rediscovered) by Harry Potter, when he finally understood that Dementors represent Death incarnate. His empathetic desire to protect all of humanity from the pain of that loss allowed him to not just drive away the fear of Death, but to conquer Death itself. This caused his Patronus to evolve into its true form; his Patronus took on the shape of an androgynous human. In this form, the Patronus gains additional abilities, including the ability to destroy Dementors and block the “unblockable” curse, Avada Kedavra.
(source: https://hpmor.fandom.com/wiki/Expecto_Patronum)