The following can be used as a summary for everything regarding Servants arriving at Chaldea, whether it be by summoning or by alternate means. All information collected here can be gleamed across the series and can be used as a refresher for those caught up.
It shouldn’t need to be stated, but because of this, everything henceforth is filled with spoilers. You’ve been warned.
The regular method of arrival. Every time FoC’s Fate System is activated, it has a chance of bringing forth a mystic code, craft coin, or calling forth a servant. If it fails to do the latter (The chosen servant refuses), it’s guaranteed to get either of the former. If the servant accepts the request, they are summoned to Chaldea. The servant is given the following knowledge upon summoning:
The Throne does not provide anything else the summoned servant doesn’t already know. Anything that is deemed ‘unimportant’ (solvable or obtainable by alternate means) will be completely neglected; There’s no need to understand the science behind ignition to be able to perform the act of starting a fire.
Example 1: Nikola Tesla is summoned to Chaldea during the Grand Orders. He’s summoned between the Fuyuki and French Singularities, so he does not have any memories of what he did during London (since it didn’t happen yet).
Example 2: Jeanne is summoned with memories of the Apocrypha War. Though she was illiterate in life, the Throne provided her knowledge to be able to read several languages for potential communication purposes. However, this doesn’t mean she can write neatly or efficiently, even in French. It also doesn’t mean she can recall and draw on any of Laetica’s former knowledge from the past war since it likely wouldn’t have made such a profound impact in her mind.
Summoning an Identical Servant
FoC’s Fate System uses a complex method of variables to perform its action, but it records both the method and variables used. This doesn’t prevent it from winding up activating a previously used or similar set of variables. However, if two variables are so closely aligned, the Fate System will not provide enough energy to complete the summoning. It will instead divert some of that energy back to the generators which will then provide surplus quantum pieces.
Though they were summoned, but not provided enough energy to manifest, the saint graph will collapse into a loose cloud-like form called a Memorial Essence. A duplicate servant that shares the same life origin can never be summoned and manifest in Chaldea. This is able to be absorbed by the corresponding original within Chaldea, where it will then bond with their Saint Graph and provide additional benefits.
A Memorial Essence can provide the original servant the following:
It should be noted there are exceptions to duplicates being summoned.
Servants who are very different from the original possess enough changed ‘data variables’ that the FoC Fate system doesn’t recognize them as a duplicate. For this to happen, the differences have to be drastic enough that they fully impact how a servant lived their life compared to the original. Common large differences that lead to this exception include (but aren’t limited to) gender, forgery, or manipulated manifestation. In these cases, the servant will fully manifest in Chaldea, and any future attempted summoning will either become Memorial Essences for one or the other.
Examples, both in series and theoretical:
Example 1: Arturia Pendragon (Saber) was summoned to Chaldea first. Arturia Lily and Arturia (Lancer) were both summoned to Chaldea, but because the original was present, they collapsed into Memorial Essences. Arturia gained access to their memories, material bodies, tools, and noble phantasms. She can use Caliburn at full potential because Arturia Lily shares the same class. However, Rhongomyniad and her mount suffer from penalties because she uses them in the wrong class container.
Example 2: Counter Guardian Kiritsugu is summoned to Chaldea. Hours later, he receives a Memorial Essence about his life during the Fate/Zero timeline. The memories clash heavily with his own distinct life and make him ponder over the new memories of a ‘happier’ time, which now feel like he personally lived. This begins to effect his outlook.
Example 3: Jeanne d’Arc was summoned to Chaldea first. Because Jeanne d’Arc cannot possibly fall to evil due to her disposition and faith, her alter form becomes an impossibility that can’t be deciphered as the original.
Thus, Jeanne d’Arc Alter is summoned and manifests in Chaldea because her Saint Graph differs wildly from the original.
Theoretical Example 1: Rama (Saber) receives a Memorial Essence with new weapon-based noble phantasms. Due to his skill Martial Blessing, he does not suffer rank down penalties if he decides to use them.
Sometimes referred to as a Rescue Operation for singularities. This is a blanket-term used to describe the alternate means a servant can arrive in Chaldea.
Any servant, whether a duplicate or not, has the chance of coming to Chaldea from a singularity. This is usually done by Chaldea attempting to Leyshift them from the the singularity with the rest of the deployed force. Because they were summoned by other means besides the Fate System, there’s zero chance they can collapse into a Memorial Essence upon arrival. They can simply be brought to the system where their ‘unique variable’ set will be recorded, registered, and tethered to the system to ensure their flow of energy. The Fate System won’t and can’t forcibly make them collapse into a Memorial Essence for doing this.
However, for a servant to Leyshift back to Chaldea, three strict circumstances must be met. If any of the following necessities is false, the servant’s saint graph will break apart during the Leyshift, and they will not appear in the Deployment Room.
Powerful livings beings who are brought to Chaldea in this way becomes Pseudo-servants if the Fate System recognizes their ‘identities’ among the many variables. Whatever the reason may be, they are considered unique servants and become tethered to the system like a regular servant. It’s a unique ‘ascension’ that brings them to an existence further from true humans, but not quite servants.
It’s obvious, but will still be noted, that anyone brought back this way will not receive any knowledge or Information from the Throne of Heroes while being tethered to the system or in transit via Leyshift. This can mean servants brought back from a singularity may potentially no more or much less about certain topics than the ones brought to Chaldea by the Fate System.
Examples in Series:
Example 1: Jeanne Alter Santa Lily (Janna) was summoned in an unusual singularity. She is a botched manifestation by Caster Gilles, so her fighting skill, abilities, and knowledge are terribly askew. After helping Chaldea secure the grail, she was in absolute panic that they were going to leave her there. Though Chaldea didn’t know all the requirements at the time, all three were met and Janna was successfully rescued. Unfortunately, due to her botched summoning, she doesn’t have anywhere near as much knowledge about other servants and concepts. She also lacks the diverse communication skills the Chaldean summoned servants have.
Example 2: Arturia Lily failed to be rescued from a singularity due to having reservations about whether or not she will be helpful. In a future singularity, the Chaldeans were able to remove any doubts or reservations she may have had, ensuring her saint graph would not dissipate during the Leyshift. The fact the original Arturia received the Arturia Lily Memorial Essence nearly a year ago is not an influential factor.