Corroded Elizabeth

Finally, the Dark Star reveals her true nature.
Corrupted by the Void Cell, she becomes a Servant of the Umbral Star. Her true identity is non other than Elizabeth Bathory.
In the first stage of Nero’s arc, even though she is technically acting under Tamamo’s orders, Elizabeth tries to leverage the state of chaos in SE.RA.PH to carve out her own domain (country). Without permission, she converts a part of Nero’s territory into her own and uses it for a concert venue.
When you concisely sum it up, it’s obviously a pretty stupid plan.
Predictably, she is immediately defeated. While throwing a tantrum about losing to Nero simply because she does not possess a Regalia, she runs into the mysterious Servant Archimedes. It is he who directs her to the corrupting influence of the Void Cell, and she sinks into its depths.
Afterwards, she plots to get rid of Nero, Tamamo no Mae, and Altera, so that she can steal everything when the dust clears.
From that point on, she continues to see Nero as her spiritual rival.
Tamamo is viewed as an intrusive evil who needs to be removed.
The Main Character remains her beloved “Piggy.” At least some things never change.
Elizabeth: “Yes! I’M the one destined to rise to the top, not some fresh-faced giant or some reprehensible old geezer Servant! I’mma conquer the Moon Cell, swipe that Regalia, take Altera’s power, and become a perfect idol! I’M the one who Little Piggy is going to fall head over heels for in the end! This is my chance, now that I’ve totally fallen off the righteous path and become the evil blood maiden, Corroded Elizabeth Bathory!”
Everyone else: “You thought you were on the side of justice until now?!”
Of course, not a single one of her plans come to fruition. Defeated, she grumbles in frustration.
Elizabeth: “No…! I was just a step away… I was so close!”
Everyone else: “No you weren’t, Liz… In fact, you never even managed the first step!”
Ironically enough, it was Archimedes’ gifts to Elizabeth, which empowered her to make a poorly thought out play for power, that ended up thwarting his own plan.

Fate/EXTELLA material: Encyclopedia of Fate EXTELLA