Introduction to World Tendency System
In Demon's Souls, your actions in the world of Boletaria will weigh heavily against your character. This should be familiar to all you gamers out there—maybe you've experienced it in the form of “morality,” “karma,” or by another name before. In the world of Demon's Souls, it's known as Tendency, and it's attributed to both your character and each of the regions you'll traverse.
When you begin play, both Character and World Tendencies start off at neutral. Depending on your actions, they can shift to white or black (good deeds make it go white while). But of course, it isn't as simple as being either black or white, as there are shades along the way that determine the level of Tendency's effects. So if you're just a little evil, enemies will only be a little more difficult to down, whereas if you're a true miscreant, well then you've gone and made an already brutally difficult game a lot more brutal.
At any given time you can jump into your Main Menu and navigate to a convenient Tendency tab to try and gauge what state your actions have left the world and your character in. Each region's Archstone is there to represent its respective World Tendency, whereas Character Tendency is represented by a human figurine in the center. For both representations, they'll gradually shine with an increasingly brighter glow or shift towards a darkness void of any light to depict white and black respectively.
As stated before, each Archstone region has its own World Tendency. These are all completely independent of each other; your actions in one Archstone can never affect the Tendency of other Archstones, and there's nothing to stop you from simultaneously having one Archstone be a shining beacon of hope and good will while another acts as a blackened bastion for all things unholy. You'll want to use this to your advantage, since you might find greater benefits in keeping a certain region white while the allure of turning another region black draws you.
In worlds of black Tendency, you'll find that enemies give you a lot more trouble than usual but also give you more Souls and better drops once defeated. That former point is the result of their attacks being stronger and your HP being lower. When World Tendency is white, the opposite is true: enemies aren't as difficult, your HP is at its max potential (depending on form, of course), but the rewards for putting down enemies aren't as great as they could be. And as mentioned before, the degree of these effects will grow with how white or black the region is.
So now that you know what you can strive for, here's what you need to do to get there. When you defeat a boss within a world, it'll shift Tendency towards white. If you lose your body within a region (which is to say, you die while not in Soul form), it'll shift Tendency towards black. These are your most basic ways of changing World Tendency, and your only options to initially stray from neutral Tendency. Once you've pushed things enough to white or black, new options will open up to help you shift back in the opposite direction. For example, in regions that have achieved a certain level of black Tendency, a special Primeval Demon will appear. Killing it will help shift Tendency back to white. This is the more basic example, however.
Each Archstone region has a special NPC that is deeply rooted in World Tendency. At the further end of white World Tendency, you'll open up access to this NPC, and will find yourself rewarded with typical RPG goodness: new quest, new item, etc. However, should you not be happy with your white World Tendency, you'll find that offing the NPCs spawned by it is quite convenient for taking things the other direction. Should you blacken the region enough, you'll find yourself greeted by its special NPC once again, only you might notice they have a different aura about them this time... Now they're a Black Phantom NPC, and like most characters in Demon's Souls, they want you dead. As you might already have guessed, killing these Black Phantom NPCs can greatly shift World Tendency back toward white. As an aside, whether the NPC you're killing happens to be good or evil, you can always count on their corpse to yield new, usually rare goodies, which will differ depending on their friendliness at time of death. It's incentives like these that'll ensure you won't simply leave World Tendency be.