Fionn mac Cumhaill is a Lancer-class Servant able to be summoned by the Protagonist in the Grand Order conflicts of Fate/Grand Order. He is also the Servant of Darnic Prestone Yggdmillennia in the Third Holy Grail War of Fuyuki.
[Spoiler] Lancer's True Name is Fionn mac Cumhaill (フィン・マックール), 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 that established many valorous deeds and miracles, such as gaining control over healing water from the oil of the Salmon of Knowledge, having saved the capital from the wicked fallen Divine Spirit Aillen that manipulated flames and sleep, defeating invaders and monsters as the guardian of Erin, and defeating in battle various and powerful wicked monsters such as the fallen Divine Spirit Aillen, the magical Demon Boar, the horse of the underworld, and in the end, even his divine ancestor, the War God Nuada.
Serving the High King of Erin, Cormac mac Airt, Fionn was a man who attained appropriate achievements to be celebrated as both the leader and greatest knight in the glorious Knights of Fianna. He underwent an impeccable journey as a great hero, but during his personal life, he also had a fate of trouble with women. His first hardship was becoming charmed by a pair of beautiful sisters and receiving a curse. His misfortune continued afterwards; seven years were stolen from him by a conflict with a fairy over his first wife. [/Spoiler]
[Spoiler] 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. His poet son Oisín and warrior grandson Oscar, as well as his loyal knights, including Diarmuid, were all in attendance. 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.
After Diarmuid fought through all of the obstacles, Fionn was the one most heartbroken through 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 there being a well of spring water nine steps away from which Fionn could produce healing water. Despite him 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. As of 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, and before long, the Knights of Fianna divided into two sides, causing a war where Fionn lost his life in the middle of the conflict. [/Spoiler]
