Living matter reservoir

The  were featureless protoplasmic humanoid creatures who had been stripped of their chemicals by the singing crystals. While once normal biological beings, the crystals used them as sustenance for the biological traits needed to become sentient and powerful. The crystals kept them in a cave.

The crystals once breached the Doctor's TARDIS and attempted to turn the crew into living matter reservoirs. (PROSE: The Singing Crystals)

One of these creatures was among the aliens trapped on Denossus. (COMIC: Land of the Blind)