The vanguard of the Umbral Star. Velber sends these to conquer planets.
Like the Moon Cell that designed high-level information life forms from human civilizations (the Servants), the Umbral Star designed organic life forms that could efficiently destroy civilizations — known as the “Anti-Cells.”
These land on planets that have been marked for harvest and use a range of methods to ravage the civilizations there.
All of the Anti-Cells have an idiosyncratic approach to completing this task. They grow by draining the spiritrons from intelligent life forms, and starve and die once all intelligent life has been destroyed, as that is their source of food.
As the Umbral Star’s harvesters, they prioritize its commands above all.

Fate/EXTELLA material: Encyclopedia of Fate EXTELLA