“Velber” is the name of the predatory Umbral Star, also known as the Harvest Star. Like a comet, it flies on a set orbit through space, passing through the Milky Way galaxy once every 14,000 Earth years.
Its appearance resembles a crying eye.
It destroys all intelligent civilizations in its path.
It has passed through Earth’s orbit once before, at which time it destroyed 80% of the Moon Cell’s data.
It is said to be a different type of observation device left behind by the same alien civilization that created the Moon Cell.
The true motive of the Umbral Star remains unclear, but during its previous harvest on Earth, the Moon Cell did get a chance to analyze its structure.
Within Velber is a central part that could be considered its core, surrounded by numerous layers of “Arks of the Stars” [individual space vessels] acting as its armor.
If one considers the main body of Velber to be its central core, the orbiting sections are its various self-made invasion weapons. Each layer of armor has its own distinct means of invasion methodologies (civilization).
Velber’s form strikes an optimal balance between offense and defense, derived from its experience harvesting a diverse range of civilizations.
The Ark of the Stars that carried the “White Titan” Altera to Earth was only one of three Arks that came to Earth in the previous cycle. The other two, the “Big Brother” and “Little Sister” of Altera, are not mentioned in this game.

Fate/EXTELLA material: Encyclopedia of Fate EXTELLA