Doing so unlocks its legendary incantation at the Cathedral of Dragon Communion, which in turn unlocks the secret of its identity.

Source: https://screenrant.com/elden-ring-caelid-white-dragon-greyoll-dragonbarrow-farming/

Eric is so curious to read the book that he cuts the barbed wire encasing it, ignores the all-caps writing on multiple pages that warn to leave the book alone, and proceeds to read aloud the evil incantation.

Source: https://screenrant.com/dumb-movie-decisions-got-characters-killed/

To do so, he recited an incantation, which came into effect by altering his own shadow into that of Mimihagi itself, after which the shadow spread out across his body.

Source: https://gamerant.com/bleach-the-soul-kings-right-hand-mimihagi-explained/