World Modification
There were five strongest races within the medium world, but two of these had disappointed the higher world's leaders. These people were treated worse than slaves, but the worst treatment reached the strongest representatives from medium world races.
They were Rabbit Blue and Raven White races.
These races' characteristics and origins were simply disappointing and so the leaders just took their strongest people to turn them into energy fuels.
Their whole bodies were producing the energies nonstop for the higher world after such 'operation' and they have been staying in the hidden basements ever since such fate fallen onto them. Their lives were worse than death as they even felt pain.
The way the power regenerates depends on the races, but also on the individual.