Altera (Earth)

Altera is a fragment of the White Titan’s brain structure, protected in its remains and left as a backup when the White Titan crumbled to the Earth.
Elders of the Hun tribe, excavated the ruins and found that fragment, in the form of a human girl, took her in and raised her.
She grew up living as a Hun, believing herself to be human (as her appearance is human).
In the world of Fate/EX, she is the one who later became king of the Huns.
Attila is said to have lived in the fifth century AD, and she carved a great empire through much of the Western world
Attila the Hun.
The Huns were descendants of the Xiongnu (Hsiung-nu), and Attila the Hun their warrior king.
Leading armies on horseback, she ravaged battlefields. Her empire was a vast territory consisting of western Asia, Russia, Eastern Europe, and Gaul, conquered through immeasurable bloodshed.
It is even said that she was responsible for the fall of the Western Roman Empire.
In life, she knew war and little else. She was called “punishment from God” and “the Scourge of God” in all corners of Europe, especially Northwestern Rome. The sight of her ferocious figure on the battlefield was widely feared.
She never knew that her drive to conquer and destroy came from an ideology embedded in her very DNA. That being said, Altera on Earth had no other connection with the Umbral Star. She lived and died as a human, revered as a hero.
From the perspective of the Altera sleeping on the Moon, the life of Attila was but a fanciful dream. This is similar to the circumstances surrounding Saber and “The Forests of Camlann and the Holy Grail War” in Fate/stay night.
But Attila’s memories left their mark on Titan Altera, sparking a glimmer of humanity within her. As a result, she began to question “her programmed mission.”
The Altera in Fate/Grand Order is the Heroic Spirit form of the Titan Altera.
Although the Altera of Fate/GO is no longer connected to the titan version of herself, she recalls traces of that connection, which she considers memories of a previous life she can’t quite grasp.
…But a vestige of her old self’s power is still buried in the depths of her core.

Because of those memories, she became aware of the “War God.” She asked the elders a question:
“Does the War God have a name?”
The elders replied that there was a War God named Mars in the Western lands. “Then my blade is the Sword of Mars. I think I would like to name it that.”
Despite not having any of her memories as Sephyr, she decided by herself to name her sword as such.
And as hero of the Huns, she led her people to march across the Western lands, establishing herself as the “great king of destruction.”

Fate/EXTELLA material: Encyclopedia of Fate EXTELLA