The Holy Grail War (聖杯戦争, Seihai Sensō) is a competition that decides the ownership of the Holy through intense battle royale. While there have been many conflicts over supposed Holy Grails in the past, this term refers to those specifically based around Masters, usually proficient Magi, summoning Servants, Heroic Spirits brought forth as Familiars, and meeting in battle until only one pair is left to claim the Holy Grail.
Einzbern, Tohsaka, and Matou(Originally Makiri) were the three magi who first started and created the battle that they are now called 'the Three Founding Families'.
The original purpose of the ritual was to recover the Third lost by the Einzberns, the "Cup of Heaven." Around 1700, Justeaze Lizrich von Einzbern, Nagato Tohsaka, and Zolgen Makiri created the system in order to create a gate leading to Akasha. At that time, the Mage's Association and the Church were locked in a battle to the death, so a land in the Far East, where the Church could not monitor, was chosen. The Einzberns provided the alchemy to create it and prepared the vessel for the Grail, the Tohsaka provided the necessary land and called forth the Servants, and the Makiri assembled the magecraft to stabilize the starting materials and designed the Command Spell or Seals to command the Servants. Back when the Tohsaka were developing the ritual of the Heaven's Feel along with the Makiri and the Einzbern, Nagato's daughter played a greater role in the completion of the Holy Grail War system than her father.
The Holy Grail War itself was devised by Justeaze, and she was to become the key of the ritual. The Greater Grail was created through forming a large circuit from many multi-layered Crests carved onto a bare rock over fifty meters in diameter. It was covered in multiple layers of circuits, while its many geometric figures turned in place. Justeaze stood in the center of the it, and became the key to reenacting the Third Magic. The Greater Grail would have only been just a Holy Grail without her, as her will was required to enact Heaven's Feel. Kischur Zelretch Schweinorg acted as a witness to the event.
Speaking about Old Man Time, the Magician of the Second True Magic was doing his duty, watching over several of the insurmountable parallel worlds in his Favoured chair and a smile. Currently he was watching over The Epilogue of Earth, one of which he found most amusing as it's Fate was almost sealed due to a certain Entity only to find out there were traitors within the organization to then came the plot twist that they wished for humanity's reset by slaying Gods to then become a Clusterfuck of Singularities their "Last Master" of Chaldea had to fix, Events that transcend and mess with the Spiritron core of each and every Heroic spirit that was present, Oh the Joy, though it wasn't without it's downfalls such as erasing Timelines
and worlds within said timelines to preserve the 'Current' Reality. Although it was nothing new to the Old Man, he found it vexing that The Holy Grails some Parallel worlds and Universes had to fight for dear life were merely given out for free.
"Grand Order." He Muttered to himself as he watched intently the current events to it, being the Tunguska Sanctuary, and began to devise an amusing scheme. Swiping his fingers around the endless Multiverse he looked for a 'Fresh Canvas' to 'Paint' his Colorful Idea. "A small Rewrite of history here and there, a Supercharged Cup here, a Dragon Vein... EVERYWHERE," The man paused and looked to a certain Ruby Wand and Grinned. "Sentience to said Cup, Super Random Summons there, A Tinge of the Secretive Chapters(Apocrypha) mechanics and Voila!"
"Fate/Myth Reign." He heartily chuckled at his 'Piece', and with a tap on the parellel world the Saga Begins....
"On Second Thought." He Waved his Hand Producing a Golden Card, to which had an image of a woman with scales and a sword, The Ruler Class, a Servant who had little to no desire for the Grail to oversee the war with little complications, Since his choices, were borderline insane... Until he had his own inside joke to it.
A quick slice produced a bright light that enveloped the room and in it's place a Man whose clothes reference of a warmer climate, with silk and cloth of the highest quality touching the form of the blonde, A Clay Tablet on one hand and a Large Lance Like sword weapon on the other. A simple Crowned Turban adorned his head as a Ruby centered it's prominence.
"Ruler Class, Wise King, Gilgamesh." Gilgamesh Identified himself as he opened his eyes to reveal shining Ruby Orbs tempered with Age and experiences. "How May I be of Assistance– YOU!"
Gilgamesh was Outraged as one, he was faced with the Man whose Involvement while minimal was detrimental to every Dimension he had visited. With His Tablet and Ea Glowing ever so violently aimed at Zelretch. "FALL! OLD MAN TI—"
With the Sound of A Pop echoing the Room The Pissed King was gone.
"A few more moments and I had to rework This World, but for now." A Grin formed itself as he sat on his Chair. "Let the Saga, Begin."