Scattered across the land are ley lines that exude a spiritual energy. Where two or more lines intersect, a nexus is formed. These nexuses are vital to the continued survival of the humanoid races, for those who do not visit a nexus at least monthly quickly wither and die.

Old age as we know it in the real world does not exist—people who diligently refresh themselves at a nexus every few weeks have been known to live for centuries or even millennia, barring violent or accidental death. However, those who fail to do so age the equivalent of one real-world human year for every month of withering. To be clear, children grow into adulthood regardless of whether they refresh, but (e.g.) a six-year-old child who has never refreshed will have wrinkled skin similar to a 72-year-old man.

Once you wither, the effect is irreversible, though visiting a nexus regularly halts further withering. People who die from the withering always rise again as undead. What kind of undead depends on how "epic" their death—as an example, a child held captive in a cellar until she withers to death might rise as an angry ghost and haunt her former captor. And in general, high-level characters often become powerful intelligent undead, while low-level ones often become zombies or ghouls.

Culturally, the withered are looked upon with pity and disgust similar to the homeless and mentally handicapped. Because it is possible that such people may wither further and rise as undead, they are also usually more feared and distrusted than other destitutes.

In recent times, disagreement over how the nexuses should be used has led to increasing tension between the major cultures.

Appearance

Ley lines appear as small grooves in the earth, which glow with a white energy visible to the eye. You can see the energy flowing—typically the direction of the flow indicates the closest nexus, though the larger the nexus the stronger the pull, so it may be that there is a closer but smaller nexus in the other direction.

This spiritual energy behaves like the more traditional positive energy of D&D in many ways, though there are some differences. But "spiritual energy = positive energy" is a good rule of thumb for most purposes.

As a ley line approaches an intersection point, it widens until it is more like a stream or river in the earth. If you were to stand inside, you would feel the spiritual energy coarsing over you, though it would not physically move you like wind does. The nexuses themselves are swirling masses of spiritual energy. They range from the size of a beanbag chair (i.e., a five-foot square) for a very small intersection between two minor ley lines, to the size of a house (perhaps 50 feet in diameter) where many or major ley lines intersect.

The ground at a nexus is suffused with spiritual energy. So it doesn't really seem barren, but neither is there usually much vegetation growing there. Though there could be exceptions to that in some cases.

There is a skill called "Geomancy" relating to the ley lines and nexuses. It allows you to determine the size of and distance to a nexus by looking at the flow of a connecting ley line. It also gives you a general intuition for the nexuses and their relationship with the flow, including allowing you to experience prophecy while within a nexus.

If you dig into the earth at a ley line or nexus, you will find that the spiritual energy continues downward into the earth. So the "ley lines" are really more like "ley sheets" but that doesn't sound as cool. :-)

Cultural control

Regarding how much control each culture has over their nexuses:

Technoprophets: Each city is built around a major nexus. They typically don't care so much about minor nexuses. The nexuses are very well protected by technomagical walls, but individuals, especially nomads, are generally allowed to visit and refresh as desired. However, the technoprophet researchers do tend to build domes within the nexus area to facilitate equipment storage for their experiments, as well as magical item production.

Nomads: They travel, and do not establish much of a presence around any nexuses, though major nexuses in well-traveled areas probably have clans visiting more often than not. Unfortunately for the nomads, more and more such areas are being appropriated by either the technoprophets or the Empire.

Empire: The Empire guards a conquered nexus with extreme paranoia and caution. They sequester it, completely walling it off so that no one but the high priests (who are tasked with destroying them) may enter. Visitors to the Empire are generally permitted, but are not allowed within a nexus perimeter for any reason. The Empire has elite guards responsible for protecting the nexuses at all times until the rituals needed to unweave them are completed, which can take many years depending on the size of the nexus.

Tribes: The tribes establish territory around a nexus and protect it as best they can from competing tribes. Bloody wars are fought amongst the hordes for control of key nexuses. More recently, the Empire has begun a major effort into traditionally tribal territory, capturing many nexuses. The technoprophets have also managed to sneak in and capture at least one major nexus deep within tribal territory using a combination of clever illusions and technology.