Humans native to Ru'un have very long lifespans, and as a result require a special type of organ to store spiritual energy. This organ is called a Keyek, and can be found on a number of different places on the body. Once a Ru'unian human obtains a certain amount of power (like an old-aged brain), the Keyek will start to deteriorate until the human can no longer continue to live. Once deceased the leftover energy is recycled back into Ru'un to be distributed to new life.
Earth-born humans are not born with any kind of limitation device on the amount of spiritual energy they can weild. However, the planet Earth is spiritually dead, with the vast majority of Ru's spiritual energy being transferred to Ru'un. Because of the lack of spiritual energy, Earth-human lifespans are relatively short, rarely exceeding 100 years.