The Moon Holy Grail War (月の聖杯戦争, Tsuki no Seihai Sensō?) of the Moon Cell is based on the ritual in Fuyuki. It is vastly different than the original, as it starts with 999 participants that are pared down to 128 Masters who do battle in a tournament format. All participants have had their souls digitalized to enter the Moon Cell, and in the case of disqualification or loss, their lives are forfeit due to the deletion of the data comprising their souls.
The Moon Cell utilizes the Holy Grail War structure to collect data. It was not called as such until around the 20th century, and the original conflicts did not have a name. It wanted the best data samples, so a war of survival based on the ritual in Fuyuki was the best method for it to separate the good from the bad. The magi invited into it would simply kill others and then flee, leaving a mountain of corpses. It changed upon Twice H. Pieceman's entrance into consciousness as an NPC, allowing him to repeatedly enter because he would revive as an NPC no matter how many times he lost.
Twice eventually became victorious and reached an elevated seat close to the Holy Grail, but he could not utilize the wish granting aspect of the Moon Cell due to being an NPC that would be deleted before the wish could be granted. He decided to manipulate the rules of the Moon Cell into a structured Holy Grail War, a battle of survival from which only a single person could emerge victorious. The purpose was to have a world where the limits of human potential can be pushed beyond imagination to have a proper person fulfill his wish for him. He invited many magi to participate over two years, leading to a vast increase in deaths related to hackers.
The participants are gathered together in a school, and they have all of their memories suppressed except for certain special people like Leonardo B. Harwey and Julius B. Harwey. Placed into the roles of students with different memories, they go through the same lessons, subjects, and content every day. Everything feels artificial, as students have trouble with memories and the general feel of the school. Those who figure out the strangeness are able to regain their own memories. Those like Rin Tohsaka are able to do it quickly, while some barely manage to make it. Direct interference with people without memory during the period is against the rules.
People close to figuring it out get crushing headaches and feel static that affects their vision as the illusion breaks apart. The overbearing strangeness only increases as people eventually start to disappear slowly over time, reducing the number of students each day. Only those who make the decision to look beyond the accepted and progress are allowed the right to exist. By recovering themselves, they are allowed to enter the main tournament. Overall, thirty percent manage to overcome it, allowing them to move on to the next stage of the preliminaries, while any who remain in the school are disqualified.
The final part grants the potential Masters an Effigy, a substitute for a Servant, and tests their capabilities. They are pitted against another Effigy, and if theirs should happen to be defeated, as with the defeat of an actual Servant, they will be disqualified. Those that manage to win will become one of the 128 competing Masters and will be granted a Servant. While Hakuno Kishinami loses, their will to survive causes their selected Servant to save them.
Those 128 who become Masters and obtain Servants are allowed into the main part of the Holy Grail War, an elimination tournament that lasts over a period of seven weeks. The last Master and Servant pair left alive then get to claim the Holy Grail as theirs. The main area is a new Reality Marble identical to the school used in the preliminaries, and all of the Masters and Servants are able to utilize it at once. They are not allowed to fight, but they can have their Servants materialize. Each pair is granted a Private Room in which to rest, and they have access to a shop, library, nurse's office, and church for their various needs.
Each round randomly pairs all of the Masters against each other, and they have a time period of one week to prepare for battle. The Preparation Period grants access to the Arena for six days, and the seventh day features the Elimination Battle. The arena, accessed at most once per day, allows Masters to gain strength by fighting the programs randomly generated there. If the programs overwhelm them, they will be disqualified. The arena generates two Triggers, a primary cipher key and secondary cipher key, for each Master that must be obtained for the right to participate in the Elimination Battle. Should they fail to obtain both, that Master will be unable to battle and immediately disqualified on the seventh day.
Aside from gaining strength, the opponents are given time to search for clues revealing the class and identity of the opposing Servant. Both pairs are allowed within the Arena at once, so they are able to interact. Direct battles between the parties are not allowed, but SE.RA.PH takes time to intervene. They can battle for a short period without worry, and it is possible for one side to lose should they be fully defeated before SE.RA.PH intervenes. Obtaining the two triggers and surviving the six days allows the opponents to enter the Coliseum to do battle, and the winner moves onto the next round.
While combat is banned on the campus, only Masters are restricted entirely. Servants may take action if they are willing to accept the penalties from SE.RA.PH. Only actions that threaten the Moon Cell itself are completely restricted, even to the point where a Servant may be forced to turn on their Master or risk being deleted entirely. For those with Mad Enhancement, SE.RA.PH will make an exception. SE.RA.PH will send a proxy program to punish Servants and Masters who are held accountable for the Servant's transgression.
Hakuno Kishinami faces the following opponents:
- First week: Shinji Matou and Rider
- Second week: Dan Blackmore and Archer
- Third week: Alice and Caster
- Fourth week: Run Ru and Lancer (Rin route) or Monji Gatou and Berserker (Rani route)
- Fifth week: Julius B. Harwey and Assassin
- Sixth week: Rani VIII and Berserker (Rin route) or Rin Tohsaka and Lancer (Rani route)
- Final week: Leonardo B. Harwey and Saber
- Extra: Twice H. Pieceman and Saver
Obtaining entrance into the core of the Moon Cell, the Holy Grail of the Seven Heavens, allows a person to harness the power of the Moon Cell to grant their own wishes.
The Sakura Labyrinth (サクラ迷宮, Sakura Meikyuu?) is created by BB in the Far Side of the Moon Cell during Fate/Extra CCC. It takes Masters and Servants from the Holy Grail War, allowing them to keep their contracts, and places them within her world.
- Chapter 1: Floor 1st - 3rd
Rin Tohsaka and Lancer
- Chapter 2: Floor 4th - 6th
Rani VIII and Lancer
- Chapter 3: Floor 7th - 9th
- Chapter 4: Floor 10th - 12th
Jinako Carigiri and Launcher
- Bad End
- Chapter 5: Floor 11th - 15th
Shinji-Tank and Shinji Matou
- Chapter 6: Floor 16th - 18th
- Chapter 7: Floor 19th - 20th
Kiara Sessyoin and Caster
- Secret Dungeon
Rin Tohsaka and Archer
Kirei Kotomine and Lancer
Twice H. Pieceman and Caster
 Fate/EXTRA material - Encyclopedia: Holy Grail War [Circumstances], p.194
Holy Grail [Circumstances]