Imma post it here if you can't acces reddit. This was made by Waifuhunter.
Yes it sounds weird I know, but after seeing most of the stuffs in Link, I came to realize that while the game is not story-heavy, it does help quite a lot in term of lore, especially for FGO. I will not spoil anything about the story of the game and will be as vague as possible about it, so don't worry!
1) Servant Saint Graph (Spirit Origin). In Link, Charlemagne asked Tamamo about a certain blue item they got in a stage, and she explained to him that it's saint graph data of a servant. It is made out of something called the Blue Particle (Not sure if it is a hint to Blue - the 5th true magic). This Blue Particle is supplement material to form servant's body. To summon a servant out of it, you need a huge amount of magic energy. Magic energy basically form your servant's body base on the data which is made out of this Blue Particle.
Now if you look at
this item
- the Blank Saint Graph which you can trade for any SSR you want, see the Blue Particle there? Bingo!
2) Shadow Servants. Yes they are in Link. As long as you have the saint graph data of servants, you can pretty much summon them anytime given that you have a mana source. Link explained that Shadow Servants are servants with vastly degraded saint graphs due to the lack of mana supply as they were created from the saint graph data. It might be due to various reasons, such as from a skill or a NP of a servant (like how Tamamo summoned Shadow Tamamo Lancer and Shadow Tamamo Cat in Nezha interlude, I guess that's why fox is expert in this lore lol).
3) How Chaldea get to summon servants that are impossible for the Fuyuki grail system. Chaldea analyzes the saint graph and record the data of every servants they encountered in the journey, including the strange ones that Fuyuki grail can't summon, in the form of the cards we have (in hindsight it's like the arcade cards for some Sega games i.e Kancolle and FGO Arcade). So whenever we need to summon a servant we saw b4 in events or singularities, Chaldea system will charge mana to the saint graph card it recorded thus recreate the servant from that data. By using this method in Link, Tamamo uses her NP to charge mana (it can provide unlimited mana for a limited time) to a saint graph data and revived a servant from it. Hakuno also used this method to do something that might in turn help bring something to FGO later down the line.
And that's it. The big mystery about why we have servant cards, why Chaldea system powercrept the other summoning systems (except Moon Cell) are all explained!
