In Fate, can a Master summon an Extra Class Servant during a Holy Grail War, or under any circumstances?

Question

In Fate, can a Master summon an Extra Class Servant during a Holy Grail War, or under any circumstances?

For example, one Master summons Jeanne of the Ruler Class, and another Master summons Edmond Dantes of the Avenger Class. Can that happen?

Answer 1

If I remember correctly, in the Fate/Zero and Fate/Stay Night universes, Avenger Angra Mainyu was summoned by the Einzbern in the third Holy Grail War instead of a Berserker. This would imply that it is technically possible to summon an Extra Class Servant during a standard Holy Grail War following the Fuyuki formula.

Answer 2

No, it was stated that the Einzbern's were only able to summon those extra class servants (avenger Angra or ruler Amakusa) because they were one of the families to create the holy grail war system, so they were able to interfere with the system to allow the ruler and avenger class to be summoned, but only by them who knew how to and had the authority over the system to do so.

Therefore if any other master attempted to summon an extra class servant they would not be able to because of the rules of the holy grail. So no, in a holy grail war, a master can not just summon those extra class servants.

Answer 3

For the first part of your question the answer for most Masters would be a no. The Fuyuki Holy Grail Wars were built to have one Servant each of the seven standard classes be summoned but since the Einzberns were part of it's creation they can to an extent go "fwahahahha I am the rules" and were able to summon an Avenger/Ruler class instead while Medea's skill at magecraft and older mages generally being far superior to modern ones was able to bend the rules enough to add Sasaki Kojirou albeit by having to bind him to specific location.

The Great Holy Grail War is an extension of this system with two of each standard class being summoned by Masters.

Outside of a Holy Grail War summoning a Servant is nigh on impossible but if achieved would not be bound by class restrictions as demonstrated by the Faker class Servant summoned by Doctor Heartless in Lord El-Melloi II Case Files using an imitation of the Holy Grail.

Fate/Grand Order also demonstrates this as both the Chaldea summoning system and the Grails found within singularities are capable of summoning Extra class Servants just as easily as any other.

Answer 4

All rules are made to be bent, broken, and superseded

This is kind of the fundamental thrust of the Fate series (or more broadly, the "Nasuverse"). Rules are more like guidelines and suggestions. Like if we were going to play a game to settle a dispute, and I put everything for Connect Four on a table, but you were like "first one to toss a token onto their head wins!" The suggestion was that we play Connect Four, but you took what was available and made it into what you could.

The Holy grail war has a system of rules and functions that it ideally operates by, but every war becomes about who can manipulate and break the rules the best. In Fate/Zero, Melloi splits the command seals and energy burden between himself and his wife, respectively. In the third war the Einzberns manipulate the system to summon an Avenger, thinking it'll assure them victory (in the alternate timeline of Apocrypha, they summon a Ruler instead). In the same war another team uses their special rule-breaking crest to basically have two masters with their own copy of a servant. In Fate/Stay Night Caster exploits the system to become a master herself, create a fake Assassin that defied the normal rules for an Assassin, and to spawn camp the lesser grail. She even has a Noble Phantasm called "Rule Breaker" that pretty much does exactly what it says. A reality marble, like Unlimited Blade Works, explicitly involves the user rejecting reality and substituting their own. In Fate/Apocrypha the multi-team grail war is activated by Clock Tower members after they infiltrate the base of those who had stolen the Greater Grail; it wouldn't normally have happened. Heck, even the existence of the Greater Grail, and its true purpose, was originally only meant to be known to the three houses that developed the idea, with every other participant intended to be under-informed schmucks that were supposed to be easily dispatched because they didn't have the advantages of the Makiris, Tohsakas, and Einzberns.

Even the combat system devolves into who can break the rules better. An unstoppable projectile meets an impenetrable shield...what happens? Well, that can literally happen in this setting, and there's an answer to it, and it mostly depends on the level of their divine mysteries and their compatibility. If this shield specifically did, or was crafted to, stop the unstoppable projectile in question in myth, then it'll easily do so here. If the user of the projectile was cursed to have his attack blocked by someone of a certain lineage, then if the shield's user is of that lineage then it will win (the shield might not even need to be otherwise "impenetrable"). But a barely known shield on a minor hero with no relation to the well-known attacker with a well-known spear will be shot through (weak mystery vs strong mystery). In Fate/Stay night this exact situation occurs in one route, resulting in a near draw (but advantage to the attacker). In Apocrypha we see that the nigh-total immunity of Siegfried can be punched through by attacks with sufficient divine mystery relative to that of his own protection, and attacks by heroes or weapons with a legend specifically for slaying dragons will be highly effective against him. Several of the main characters of Apocrypha are in fact intrinsically warping and defying the rules, both of the Grail War and basically existence itself.

So, sure, all sorts of things could be summoned, provided the Masters and others involved are sufficiently skilled and clever enough to manipulate and break things in the right way to make it happen. And in Apocrypha there are in fact multiple Extra Class servants active at once.