A massive mansion. Wait, that’s Caesar.2

Sefar is the avatar that the Giant God Altera used when she appeared on Earth. Altera’s final form. This is the alternate form that the Altera within the Moon Cell created to destroy the Earth’s civilizations 14,000 years ago.
First, she sent her data to the Earth’s surface, and created a giant of herself that was sixteen meters tall.
Then, she sent it on a rampage as a titan, and began destorying the civilizations on the Earth’s surface.
She used the power of the Anti-Cell to make animals giant as well, and used them as tools in her assault upon the humans of that era.
Those animals, who destroyed what are now known as the Western European territories, were immortalized in cave paintings. (In the world of Extra, these are the murals found in Altamira cave.)
She spread destruction everywhere she went, and fought with all of the gods who were protecting humanity, eventually triumphing over the majority of them.
After defeeating “the concept of the War God,” she gained control of the Sword of the War God. (That is the reason why Greek mythology depicts Mars losing to the goddess Athena.)
Altera’s might forced even the gods to retreat, but in the end, she was defeated by the hands of an mere human. That human was the wielder of the Holy Sword, also known as the Divine Armament, which was forged in the inner sea of the stars.
Altera escaped from the battlefield in a near-death state and eventually perished in a desolate field far away. The cave paintings of the White Titan were made by the people of that land, who had witnessed her end.
After quite some time, Altera’s remains became a relic that eventually formed a cave, which was later excavated by the elders of the Hunnic tribe.

Sefar appears as the final boss in Nero, Tamamo, and Altera’s endings. Her distinctly alien appearance is quite captivating.

Translator’s Notes
  1. ^ Localized as “Sephyr.”
  2. ^ A joke referencing the similar pronounciation between Sefar and Caesar, referencing Ceasar as a “vast mansion” due to his appearance in Fate/Grand Order.
Fate/EXTELLA material: Encyclopedia of Fate EXTELLA