The Holy Grail War
A complex magical ritual devised some 225 years ago by three great families of mages; Einzbern, Tohsaka, and Matou. Its original purpose was to gather and channel the energy necessary to achieve a great feat of magecraft. The original purpose of the ritual was soon cast aside however, and it quickly devolved into a bloody fight to the death. The three great families have been bitter enemies ever since, though they have been known to occasionally cooperate when the need arose.
There have been a total of five Grail Wars so far, though little is known about the first two, and only scattered fragments of knowledge are left concerning the third.