Ophelia realizes she never had enough power to kill Priscilla, and now she has become an Awakened Being (AB) herself..
so, Ophelia "passes the torch" of her revenge against Priscilla unto Clare's "shoulders" (whom has her own burden of revenge against Priscilla, due to the death of her beloved Teresa, hehe).
So, Ophelia sacrifices her life in order to complete Clare's mastery of the Quick Sword (Clare's heart was/is an "inferno" and the Quick Sword required calmness/peace, so Ophelia used determined/resolved motivation to help Clare be able to control her Quick Sword Irene's arm)