Fionn was the unparalleled leader of the Knights of Fianna of Erin, an "almighty group of knights", and friend of Diarmuid Ua Duibhne.
When Fionn was to be betrothed to his third wife and daughter of the High King, Gráinne, they put on a large banquet to celebrate the occasion, feeling as if the engagement was more joyous than any other. Due to Gráinne begging Diarmuid to take her away from there, the two ran away together, and Fionn, filled with jealousy and resentment, mobilized all of his resources to hunt them down.
In Fate/Grand Order:
