Fionn mac Cumhaill is a Lancer-class Servant available for summoning by the Protagonist in the Grand Order conflicts of "Fate/Grand Order". He also serves as the Servant of Darnic Prestone Yggdmillennia in the Third Holy Grail War of Fuyuki.
Lancer's True Name is Fionn mac Cumhaill, the unparalleled leader of the Knights of Fianna of Erin (Ireland), an "almighty group of knights," and friend of Diarmuid Ua Duibhne. A descendant of the Celtic War God Nuada, Fionn was a great hero and warrior who achieved many valorous deeds and miracles. These include gaining control over healing water from the oil of the Salmon of Knowledge, saving the capital from the wicked fallen Divine Spirit Aillen that manipulated flames and sleep, defeating invaders and monsters as the guardian of Erin, and ultimately even battling his divine ancestor, the War God Nuada.
Serving under the High King of Erin, Cormac mac Airt, Fionn was a man who earned achievements worthy of being celebrated as both the leader and greatest knight in the glorious Knights of Fianna. His journey as a great hero was impeccable, but his personal life was marked by troubles with women. His first hardship was becoming charmed by a pair of beautiful sisters and receiving a curse. Misfortune continued; seven years were lost to him due to a conflict with a fairy over his first wife.
When Fionn was to be betrothed to his third wife and daughter of the High King, Gráinne, they held a large banquet to celebrate, feeling as if this engagement was more joyous than any other. His poet son Oisín and warrior grandson Oscar, along with his loyal knights, including Diarmuid, were all present. Due to Gráinne begging Diarmuid to take her away, the two fled together, and Fionn, filled with jealousy and resentment, mobilized all his resources to hunt them down.
After Diarmuid fought through all obstacles, Fionn was heartbroken by the futility of all the sacrifice, so he decided to recognize their marriage, bestow upon Diarmuid a title and territory, and take him as a subject once again. Though Diarmuid wished for reconciliation, Fionn could not let go of his bitter jealousy over the affair. In times of peace, he often spent his days quietly while carrying out events like hunts with his subordinates in the knight order. On one such hunt, Diarmuid was gored by a magical wild boar's fangs but felt no fear due to a well of spring water nine steps away from which Fionn could produce healing water. Despite Diarmuid being on the verge of death, Fionn could not forget his anger, so he managed to spill water from his hands during the first two attempts to bring Diarmuid the water. On his third attempt to scoop water, Diarmuid passed away from his wound.
This dispute eventually caused the glorious order of knights to collapse. After this incident, many of Fionn's subordinates were disappointed and provoked by this event, leading to the Knights of Fianna dividing into two sides, causing a war where Fionn lost his life in the middle of the conflict.
