Fate/Re:Zero - Heroic Spirit Integration System (Fanfiction)

Transmigrated into the world of Re:Zero with a Grail-gifted system that allows him to integrate the power and abilities of various Heroic Spirits into himself, Roy makes his path through the plot while aiming to become a supreme existence.

He starts with being able to bring out only 10% of his integrated Servant's power, but his Integration Percentage raises as he progresses through the plot along with Subaru, unlocking more of the Servants' powers as well as new Servant Classes.

With the unexpected guest, the story of Re:Zero takes an unexpected turn as Roy uses his knowledge of the novel and Return by Death to his full benefit.

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It is not an AU. The MC is intruding into the original world of Re:Zero, with the power to bend fate.

-Subaru is still Satella's lover and a crucial 'protagonist' of the Re:Zero World, but Roy is the true main character of this story and the events around him, due to his actions and strength. Roy shares Return by Death with Subaru due to unexplained reasons (very likely due to the Grail inside him, a defensive mechanism).

- The System allows the MC to borrow the strength and abilities of seven different Servants, making their strength his. One of the unlocked Servants would be his main one, but he can also combine abilities and strength from multiple Servants, though at reduced effectiveness (eventually being able to become stronger than a single Servant).

-The Servants that the MC receives would be rolled randomly, being ones that he's 'compatible with'.
(Spoiler about Servants unlocked, in no particular order: Saber - Mordred Pendragon, Caster - Medea, Lancer - to be announced, Assassin - to be announced, Raider - to be announced, Berserker - undecided (hopefully with Alter version and high Mad Enhancement), Archer - undecided)