Zurai said:
It's not bad mechanically (although thoroughly unfufilling, IMO), but it has plot holes big enough to drive a planet through. If the current health of the world is static, then:
Is hp damage permanent without magical healing? In other words, how do you account for natural healing?
What about non-inflict spells that deal damage? Do they siphon health off too, or does the damage just contribute to the entropy of the universe?
What about children? Are they born with zero hit points?
How about when people die of non-hp related causes (old age, for instance)? How is that handled?
Etc...
You know, I didn't think the idea through past the initial concept...but these are all issues I'd have to face if I ever actually ran this concept as a campaign.
In my mind, though, there is a clear distinction between "Divine Life Force/Healing" and "Natural Healing", with Divine Life Force/Healing being crippled.
i.e. A Troll can still regenerate, an apothecary can still mend wounds naturally, the Heal skill becomes very important, etc...
The idea here is that Evil wants to subtract, while Good wants to add. Evil adds Negative Energy to the world (in the form of Inflict), while Good adds Positive Energy to the world (in the form of healing).
We've removed the potential to add positive energy, and we're only left with the ability to "even out" the damage we do.
As you can see, the political and moral implications are astounding, and we're essentially faced with an accelerated, apocalyptic "Global Warming". In a world where Good balances Evil, Positive balances Negative...what happens when Good/Positive is removed, and the best the forces of Good can do is mitigate Negative Energy...but not counter it?
This also creates all sorts of potential for RP rich encounters.
-A wounded priest is faced with a dying child...and the only way to save the child is to potentially kill himself by draining his own life force.
-The party Barbarian races into battle (like most Barbarians do), and gets severely injured while destroying all enemies. In an act of self-sacrifice, the cleric gives his HP to the Barbarian...but now must spend several days nearly incapacitated as he heals naturally. Will the Barbarian act more cautiously now, knowing that his actions cause his friend suffering?
-An evil empire fuels its military by sending along a priest with each soldier unit...but also keeps several innocent civilians as prisoner "Life Batteries" for the troops.
-Unable to cope with the necessity to drain life in order to heal, hundreds of Exalted Good Clerics leave their faith en masse. What does this do to the power of the Good Deities?
-Many Neutral Clerics are drawn further and further to the "Dark Side" as they grow accustomed to siphoning life from others to strengthen themselves. What are the implications?
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Of course, you're very correct in stating that there are a lot of gaps in this, and I would have to work through these before starting a game in this world.
Still, though, what are your thoughts on the concept?