Velber

The Velber are an alien threat that exist in the Type-Moon Universe.

History
Around 14,000 years ago the Velber detected sentient life on the planet Earth. Three members of the species made landfall on the planet where they caused reprehensible damage to the planet. One known as Velber 02 encountered the ancient gods of humanity and proceeded to wipe them out bringing the Age of the Gods to an end. The Velber were defeated due the efforts of the user of Excalibur and also the Moon Cell. Though the Velber were not killed but merely went into hibernation, awaiting the rest of their fleet of the Harvester ship to arrive in 14,000 years.

Culture
It seems that the Velber cyclically arrive in the Milky Way Galaxy every 14,000 years to purge life.

Source

 * Fate/Extella (2016)