Void Prisoner is a boss in Final Fantasy Dimensions. Defeating him gains the warriors a piece of dark matter.
Void Prisoner can cast a wide range of black magic. The most powerful spells include Comet and Quake. He can also use Cursed Gaze and Glare to inflict statuses to one member. Gravity has the highest damage potential, which will cut one's HP in half.
The fight is not so difficult as one healer is necessary to keep the team HP high and deal with Gravity. Other members can attack him continuously without any problem.
Voids are vast spaces between filaments (the largest-scale structures in the Universe), which contain very few or no galaxies.