Here's my take on the subject of ley lines - if there's any interest, I can write it up properly and declare it as Open Game Content...
Under this system, the GM assigns a Potency rating between 10 and 100 (or higher) to ley lines, reflecting how much magical energy flows through them. A minor ley line will have a potency of 20-40, an average ley line will have a Potency of 40-60, and a major ley line will have a potency of 60-80. Only the most potent ley lines will have a potency rating above 80.
A ley line usually runs across the landscape for a number of kilometers equal to roughly half its Potency, although in rare cases a ley line may run for a distance up to two-thirds of its Potency. According to these guidelines, a typical ley line will be somewhere between 20 to 30 kilometers in length.
Some ley lines have a Potency that fluctuates in response to astrological cycles or other natural influences. For example, a variable ley line might have a Potency of 50 most of the time, but on nights when the moon is full it might temporarily 'flare up' to a Potency of 60-70. Characters can use Lore (Astrology) or Lore (Geomancy) to detect the most auspicious time to draw upon the power of a ley line.
The system uses a new magical skill called Attunement based on POW + CHA. Whenever an Adventurer attempts to draw upon the power of a ley line, its Potency is matched against the Attunement skill of the character in an opposed test. If you don't want to introduce a new skill, you could easily use Meditation or Persistence instead.
Spellcasters usually draw upon the power of ley lines at the nexus points where two or more ley lines intersect. These are an important strategic resource and are often under the control of a faction such as a sorcerous order or divine cult. They are particularly valuable for groups who want to perform potent magical rituals or enchant permanent magic items.
Such locations often attract spirits of various kinds who "feed" off the flow of magical energy associated with the ley lines. This is why such locations often gain a reputation for being haunted or are associated with the denizens of faerie. This is also why ancient cultures often built burial mounds and the like at ley nexus points - they believed that the energy concentrated at such points would make sustain the spirits of their ancestors and make it easier for them to cross over into the afterlife. Many ley nexus points are places of power guarded by a genius loci who may be friendly or hostile towards seekers depending upon its own nature and inclinations.
To determine the Potency of a nexus point, simply add together the Potency of the ley lines that meet at that location. Thus, a nexus point formed where a ley line with a potency of 80 intersects with a ley line with a potency of 60 would have a combined Potency of 140.
A nexus point always has a radius in meters equal to 1/20th of its Potency (rounded down). The nexus point described in the previous example would have a radius of 7 meters. The nexus points that form where several ley lines intersect are rare but frighteningly powerful - locations with a Potency of 200 or more are not unknown.
An adventurer who attempts to attempt to tap into the magical energy flowing through a nexus point must win an opposed test between their Attunement skill and the Potency of the magical forces flowing through the nexus. No more than one attempt to tap into a nexus point can be made every hour. Successfully harnessing the power of a ley nexus provides the caster with a source of Magic Points beyond their personal POW. An adventurer must be within the radius of a nexus point to harvest magic points from it.
Each turn, a ley line nexus yields a number of Magic Points equal to 1/10th of its Potency (rounded down). Thus, a nexus with a Potency of 64 would yield 6 magic points per turn and a ley line with a Potency of 88 would yield 8 Magic Points per turn.
As these Magic Points are "harvested", the Potency of the ley nexus temporarily drops by the same amount. A caster who taps into a ley nexus for several consecutive turns may reach a point of diminishing returns where the flow of magical energy gradually slows to a trickle.
Magic Points "harvested" from a ley nexus replenish themselves at a rate equal to 1/10th of the location's normal Potency per hour. This, if a ley nexus with a Potency of 80 is reduced to 40 it will take five hours to return to its normal Potency.
Obviously, harnessing the power of a large ley nexus is beyond the capabilities of most individual spellcasters. Important nexus points are typically controlled by a group rather than an individual simply because the risks of harnessing such mighty forces can be high.
When rolling to see if the group taps into the ley nexus safely, each participant can add their critical score with the Attunement skill to the skill level of one individual designated as the coordinator of the ritual. Given that the potency of a large ley nexus can be high, groups have a higher chance of successfully tapping their power without disaster - although all members of the group must be within the radius of the nexus point for the duration of the ritual (which places an effective upper limit on the size of the group).
Failure to control the power unleashed by tapping into a nexus point where ley lines meet can have catastrophic consequences - both for the participants in the ritual and the surrounding terrain. The rewards and the dangers scale in a non-linear manner with the amount of magical energy concentrated at the nexus point. It may take numerous spellcasters to harness the power of a potent ley nexus, but the amount of power available can be staggering. Of course, the risk of a backlash also grows exponentially with the size of the ley nexus. For an extreme case of what could go wrong, think of the sinking of Atlantis....
Incidentally, the "Sanctity" of a sacred site is simply raw "earth power" that has been attuned to a specific cult or deity. In pantheism and nature spirituality, this raw earth energy is venerated directly at locations where it is strong - without converting it into Sanctity tied to a specific religious cult through an act of consecration. This is also the reason why religions often re-occupy locations previously associated with older cults.
There's a bit more to the system than this - for example, dowsing can be used to detect ley lines and some individuals can use Attunement to gain various other effects while standing close to a ley line (such as removing levels of Fatigue or even replenishing spent Hero Points), but this is enough to give you a feel for the concept.
What do people think? Is this a viable approach?
