Tensei Daiseijo no Mezame: Shouki wo Joukashitsuzukeru Koto Nijuunen, Okitara Densetsu no Daiseijo ni Nattemashita

The hero's party have defeated the Demon King, who had been menacing over the entire world. But just when the Demon King dies, his body releases a toxic gas that will bring the world to ruin. Just when everyone is in despair, Saint Sophia, one member of the party, succeeds in stopping the Demon King's toxic gas by breathing it all into her own body and sacrificing herself in the process. Her holy power has crystalized, and continues cleansing the air to this day. People all around the world are grateful for Sophia's actions, and bestow the title of "Great Saint" upon her. Time marches onward, and twenty years have passed since Great Saint Sophia cleansed the world of the Demon King's toxic gas. Reborn into a bewildering world where all her former companions are now old men, can Sophia continue her life in peace? Source: Kodansha