PWD
I haven''t quite worked it out yet, but the system I set up is all about that flow of power.

In the beginning the power flows FROM the earth to beings (in the beginning all beings were neutral)
When beings die, the power (it''s not really power as much as it is lifeforce) flows back into the earth.

But intelligent beings learned that they can sometimes prevent the lifeforce from flowing back into the earth, taking some of it for themselves.

Doing this interrupts the way nature was intended, and those that do will from then on be considered ''evil'' by the planet itself and no longer gain the lifeforce they used to (when they were neutral).

There is a fine line though, between becoming evil and just remaining neutral. When for example a hunter kills prey, he does it for food. When the prey dies, most of the power flows back into the planet, but some of it is left inside the meat (and the same applies to plants, but in a lesser degree). When the hunter now eats the meat, his lifeforce grows, be it by only a small amount. This is the way neutral beings function. They eat to grow (not just to stay alive).

Evil beings usually don''t even eat the meat from their prey, they simply want to take the large part of lifeforce that escapes the body of their prey after death.

Good beings that learned all about this flow of power after seeing evil beings corrupt it, decided to make a stand and they stopped eating meat altogether (some even stopped eating plants too)

This somehow resulted in them gaining more lifeforce coming from the planet itself.

Players start of as neutral beings. They have choices to be made:
1) Remain neutral, only killing to eat. This would result in a very normal pace of gaining lifeforce.
Neutral players are highly independent. They can pretty much do what they want. They can travel the world without any obligations. Neutral players make excellent merchants as they can deal with both good and evil. And they still have the choice to become good or evil (depending on their actions)
2) Become good, by not killing for a while (which will result in a temporary lifeforce loss even, as the body starts to use the lifeforce to keep itself alive). After some time, the planet will start to provide more lifeforce to the player he will start to gain in total lifeforce.
Good players have certain restrictions placed upon themselves. First, they cannot attack any other good beings (which is at the same time a blessing, as good cities will be a safe haven for good players). Second, they cannot attack anything without first being attacked. This also applies to evil beings. Even when they detect an evil being, if that being does not attack them first, they cannot attack it. Third, they have to live their lives for the greater cause. They will be asked to perform quests, certain duties must be kept. The good side will be a highly structured society in which each character adds to the total strenght. (I see the good side as an excellent way to get some politics into the game)
PS Good players CAN still become evil depending on their actions.
3) Become evil, killing whatever the player wants and draining the lifeforce from it. The lifeforce loss will forever have to be counterbalanced by killing.
Evil players have only one restriction placed upon them: they can never ever revert back to neutral and/or good.
Other than that, they can do just about anything they want. They take extreme pleasure in killing ''good'' beings as they grant them the highest amount of lifeforce. But they take just as much pleasure out of killing other ''evil'' beings, because from them as well they can draw lifeforce. Evil society is a place where players have to be on their guard pretty much 24/7. That trusted friend you''ve been adventuring with might just decide to plunge a dagger in your back when you''re not watching.

In short:
Good: living for the greater cause. Steady supply of lifeforce but almost constant need to do good.
Evil: living for lifeforce. Kill, kill, kill... or be killed.
Neutral: pretty much everything else. Explorers, merchants, messengers, diplomats, founders of cities.

(hope this doesn''t get too long and boring)