“My specialty as a Heroic Spirit is tinkering with tools that have a bit of notoriety and whip ‘em up into Noble Phantasms... but I don’t get many chances to work on a real Heroic Spirit.
“You see, I’d need the owner’s consent for that, which is normally a no-go.
“But when the pieces fall into place, an ‘exception’ comes out tasting best.
“Basically, I’m gonna fiddle around with your ability to ‘become anyone’ and elevate it.
“I’ll make it so that you can turn into strangers even more perfectly.
“Of course, it’s up to you whether your Master counts as a stranger.”
Alexandre Dumas’ proposal had been quite a tradeoff.
It was to temporarily enhance Jack the Ripper’s abilities by combining one of his Spirit Origin’s skills, Thousand Faces, and his Noble Phantasm, Natural Born Killers, with one more “ingredient”—the essence of his Master, Flat Escardos.
It was a metaphor, of course; Dumas did not chop Flat up and toss him in a pot.
Still, for the Master and Servant whose links of magical energy Dumas strengthened and whose beings he artificially intermingled, it was equivalent to being thrown in a blender and turned into a mixture of ground meat.
For the Master, it meant having his own nature mixed with the Spirit Origin of a “killer.” It was impossible to foresee what side- and after-effects it might have. He might lose his magecraft, or unconsciously commit murders, drawn along by episodes associated with the Heroic Spirit Jack the Ripper.
There was no end to the conceivable negatives... but Flat had agreed without hesitation.
Jack had been fortified by Dumas’ Noble Phantasm and gained the ability to become the mage Flat Escardos down to the last detail, including his magecraft.

Fate/Strange Fake: Volume 5, Chapter 15 ("Gold & Lions II")