Ley

Ley energy flows invisibly around Exandria in ley lines (also spelled leylines), which can serve as sources of magical arcane power. These flows transform the environment, especially where they converge, and they affect arcane magic and the barriers between the planes.

Description
The ley lines move and change shape based on planar and celestial alignment. For example, on Winter's Crest each year, the ley lines across Exandria "cross and swell", causing the barriers between the planes to grow thin. During celestial solstices, the barriers between the planes are thinnest and many extraplanar attacks or incidents have coincided with these solstices. During an apogee solstice, it is possible for ley lines to shift much more drastically. Ley lines shift over eons in a slow but cyclical pattern and its movements can be tracked and measured using a device called a geometer, which resembles a weather vane consisting of a series of spinning concentric rings.

The primordials are of the same general nature as the ley lines, as part of the native magical energy of the world.

In the Tal'Dorei Campaign Setting and Tal'Dorei Campaign Setting Reborn, the adventure hook The Ley-Knot centers on a crisis caused by a High Shaman of the Moonsteeth gnolls who had figured out how to tie the ley lines running over Rootgarden Marsh into a knot, causing arcane magic to stop functioning within a 100-mile radius. Many arcane power sources, called arcane batteries, draw their energy from the local ley lines.

Appearance
When Keyleth used Commune with Nature, she sensed the ley lines as a misty blue energy that expanded out in a network in many directions and without a visible pattern, although she could not sense them beyond several natural boundaries. When viewed through the Veilscatter Scope, Laudna was able to see a ley line in the sky, which looked similar in appearance to an aurora. The lines show colors from all over the spectrum, shifting from dark red to purple to turquoise to yellows.

As the apogee solstice approached, these lines became visible to the naked eye in the area of the ley nexus and were getting brighter with each day. Using Hunter's Bane, Chetney was able to smell a faint buzz in the air, "like the faintest touch of a tongue to a nine-volt battery".

Nexus points
A nexus is a point in Exandria and its mirror planes of the Feywild and Shadowfell where several ley lines come together and concentrate the the strongest sources of magic power. These locations often house have portals leading into other Planes of Existence, including the nexus points at the elemental rifts guarded by the Ashari tribes. Temples, including ziggurats, are often built there to channel the necessary power for worship and religious rituals. Many magical rituals, as well, have increased power when performed on a nexus.

There are three known nexus locations: one was in Cathmoíra, at the time of the Calamity, another was marked by the Sun Tree planted at the leylines collision, as well as the third nexus point located north of Yios. In addition, Hondir hypothesized that Marquet will have three possible collision points in the month of Fessuran of 843 PD: in the Panagrip Sands, in the ocean north of the Oderan Wilds, and in the Hellcatch Valley (about 45 miles outside Bassuras), which was confirmed by Dr. Baryn Vestisho to be a proven nexus point, established in the centuries past as a focus for a solstice and reclaimed by the Cerberus Assembly in around Unndilar 843 PD.

Tishtan site
Tishtan was a nomadic, magic-obsessed society that traveled across Exandria during the Schism. Long ago, they established a site upon a leyline in the Hellcatch Valley to channel the ley energy in the time of solstices.

For about five years since circa 838 PD, this site, that became ruins over the centuries, was excavated by the Aydinlan Seminary. But due to the upcoming Apogee Solstice at about 21 to 23 Fessuran 843 PD, and the site location becoming the nexus point where ley energies overlapped, the Cerberus Assembly forced members of the Grim Verity at the Aydinlan Seminary from their excavatiion site in Unndilar 843 PD, three months before. Ruby Vanguard began building a Malleus Key there.

The Court of the Lambent Path, the governing power of Aeshanadoor that shares a border with the Hellcatch Valley to the northwest, make an agreement with the Seminary to allow the Assembly to oversee the excavation site in a loose partnership. However, closer to Fessuran 843 PD, the Assembly has slowly took hold over the majority of the operation and barred much of the Seminary members from directly delving at the site.

Age of Arcanum
During the Age of Arcanum, siphons were placed at nexuses of ley lines across Exandria to gather magical energy for the crafting of powerful artifacts, including the Vestiges of Divergence.

The flying city of Avalir traveled across Exandria along the ley lines. Unbeknownst to the citizens of Avalir, the Tree of Names grew along the ley lines and used Avalir's path to write a protective spell across the world against extraplanar threats.

Shortly before the Calamity, Laerryn Coramar-Seelie tracked an upcoming apogee solstice and learned that it had the potential to drastically change the patterns of the ley lines. Her interest in ley lines and the support they provided for major magical workings was tied to her desire to transport Avalir to another plane using a machine called the Astral Leywright. This machine would redirect a ley line out through the other planes.

The many magical battles of the Calamity reshaped Exandrian ley lines. Following the defeat of Tharizdun, Pelor planted the Sun Tree in what would eventually become Whitestone at the nexus of several ley lines. The Verdant Expanse's rapid growth and fey influences were also due to the drastic restructuring of ley energy at this time.

Vecna
Several centuries later, the cult of Vecna called the Remnants created more siphons in a bid to bring back their master. The second attempt at creating such a siphon took place in Duscar 810 PD; Delilah Briarwood had intended to wait until Winter's Crest to perform a ritual that would allow ley energy to be siphoned from the nexus at Whitestone (at Ioun's sunken temple, beneath the Sun Tree) to Vecna's city of Thar Amphala in the Shadowfell. However, she successfully performed the ritual earlier than intended due to Vox Machina's arrival in Whitestone.

The Remnants created another siphon at the location of another ziggurat, beneath the strange Smouldercrown Mountains in Marquet's Rumedam Desert. Soon afterward, the power was used in a deadly ritual that brought back Vecna.

Campaign 2
Planerider Ryn is an expert on arcane ley lines who, by 836 PD, had theorized about a possible major shift in the veil between the planes, though she indicated that she needed to make more observations to be sure.

Campaign 3
Planerider Ryn found out that the ley energies movement seemed to coincide with the recurring flares of the moon Ruidus.

Before the apogee solstice in 843 PD, Chetney once has observed this phenomenon in the past, the shifting colorful leylines in the sky that lasted a full week, but "it was a lot stronger, and farther north".

Birdie Calloway, along with her husband Ollie, worked with Ira Wendagoth and Hondir on a device to be activated at the upcoming apogee solstice. The goal of this device was to learn more about the encroaching influence of Ruidus on the Feywild, and to determine the new ley patterns caused by the solstice. Even in an unfinished state, this device, known as the Veilscatter Scope, allowed the viewer to see ley lines. Cerberus Assembly and Otohan have been building matching devices, both in the Shadowfell and the Fey Realm at the same exact nexus point.

Fresh Cut Grass's design includes a crucial cable known as a "ley tether" that connects to an arcane battery, though the tether's exact connection to ley energies is not stated.

Bells Hells found that as the date of the solstice neared, ley lines became more visible without any magical assistance. Orym noticed them as faint waves of color in the sky eleven days prior to the solstice. A few days later, they were extremely visible and appeared like an aurora, and it was confirmed that they were visible not just in Marquet near the nexus, but in Tal'Dorei as well.

Notable mentions

 * When the Island of Viscan interfered with Allura's arcane teleportation, taking her and Kima to an unknown location, Keyleth visually scanned the ocean from the beach for signs of where Allura and Kima had ended up. She happened to focus on the distant location where they had ended up, and suspected that the arcane winds that blow through the ley lines might have guided her attention there.
 * When Vox Machina asked Artagan why he hadn't returned to Exandria on his own, he said "the ley lines that guide the paths" are affected when things "rearrange every eon or so". In truth, Corellon had banished him.
 * Keyleth took a sprig from the Sun Tree back to Zephrah and tried to propagate her Raven Tree there, hopefully along a ley line.
 * During one casting of Sending, it was noted that the message traveled along the ley lines to reach its intended recipient.
 * Essek Thelyss has a device, presumably a geometer, atop the tallest tower of his estate that keeps track of the shifting of ley lines.