Void Cell

A Void Cell is a spiritron-gathering entity. Although it is of the same standing as the Moon Cell, its existence appears to be of a different nature.
This particular Void Cell dwells at the core of Altera’s Ark of the Stars.
The moment Archimedes came in contact with the Ark, he was contaminated by the Void Cell, and it was subsequently passed on to Elizabeth as well.

Fate/EXTELLA material: Encyclopedia of Fate EXTELLA