He tried using his mana cubes here, even though he could not sense any mana in the surroundings, and as expected, it did not work. They would have to rely on their old method of using mana stones as sources for a bit longer.
The question was.. did the Demon Lord know they would end up here eventually? The guy did leave them alone once Damian started to wake up from the memory trap. He had to know what all these white boxes in his own Astral Plain were.
