Leylines are currents of magical and natural energies that flow through the world. They do not appear to the normal senses, and only those specifically trained to sense them can detect leylines without magical or technological aid. Leylines are a part of the world, rather than a separate construct. They shift and flow in accordance with changes in the world and the planes, and artificial influences upon them can in turn affect reality.
Leylines come in multiple grades. More local leylines are weaker and more volatile, but are easier to tap into. Larger, world-spanning leylines carry more power but are also more stable and harder to influence.
Immediate leylines are the weakest of all leylines. They affect only their immediate area, and rarely span a greater length than a single room or field. They are more easily influenced by outside forces, and can shift and rearrange themselves rapidly. Immediate leylines are rarely the same even after a few minutes time. Immediate leylines are rolled for anew at the start of each encounter. Encounters that span particularly large areas (for example, a running battle that travels across a city district) might call for immediate leylines to be rerolled. Immediate leylines should also be rerolled every 2d4 minutes that characters remain in the same area (on average, every 5 minutes).
Local leylines span greater distances than immediate leylines, and are mildly more stable. They affect entire areas, such as larger buildings, dungeon complexes, and smaller geographical environments. Local leylines are rolled for upon entering each new adventure site. Particularly large adventure sites might have different sets of local leylines in different sectors. Local leylines should remain the same for a given area in the short term, but they should also be rerolled every 2d8+15 hours (on average, every 24 hours).
Greater leylines span entire provinces. They rarely change on their own, barring significant changes to the environments they cross. Greater leylines occasionally transform into new ones, but this process occurs over the span of weeks.
Superior leylines cross whole continents. Some span entire worlds. Superior leylines, when they do change naturally, can take months or years to show any noticeable alterations.
Planar leylines exist throughout the entirety of the planes they cross. Most planar leylines connect multiple planes together through the Astral plane, but a few exist across only a single plane, or tether one or more coterminous planes together without crossing the Astral plane. Planar leylines almost never change naturally, and the few such recorded cases have occurred over the course of centuries.
Artificial leylines, as their designation suggests, are artificially created. They can function at the size and strength of any other leyline, and are similarly difficult to tap. Unlike natural leylines, artificial leylines are not influenced by their environment and do not change with alterations of the world around them. Artificial leylines do, however, decay over time, becoming smaller and weaker until they dissipate entirely.
As the DM, you do not need to know in advance how many leylines and of what type permeate your world in order to use them. Simply roll on the appropriate table when the question comes up. Smaller, weaker leylines that influence more local areas are intentionally designed to be unstable so that they can be safely forgotten, reducing bookkeeping. Only larger, more potent leylines that influence entire adventures (and are constant across them), and artificial leylines that you yourself place in your world, need be recorded.
Leylines come in different types. Characters and creatures that are adept at using the power of leylines can tap into specific leylines available in the area around them in order to draw upon their power. Leylines usually do not have any particular influence except when being tapped.
Elemental: Air, Earth, Fire, and Water. These four types of leylines are the most common of all leylines. One or more of them is present in nearly all environments. When tapped, they enhance the power of leyline magic of the matching type, and improve resistance to magic of the opposing element.
Energy: Acid, Cold, Electricity, Fire, and Sonic. These five types of leylines are often found in appropriate hazardous environments, and are common occurrences on the sites of former magical battlefields. When tapped, they provide resistance to the matching type of energy damage, and improve the power of leyline magic dealing the associated energy type. Note that while Elemental (fire) leylines are commonly found in similar places as Energy (fire) leylines, the two are separate and independent.
Alignment: Chaos, Evil, Good, and Law. These four leylines form the moral basis of planes. While relatively rare at the immediate and local grades, alignment laylines frequently occur at the greater or superior grade when communities and nations have strong mindsets. Aligned planes always have a planar leyline of the associated alignment.
TODO: More leyline types.
- Most Cleric domains, really
TODO: Random leyline tables.
TODO: Leyline-using magic system.