Ley

Magical ley energy flows invisibly around Exandria in ley lines that enable the use of arcane magic. These flows transform the environment, especially where they converge, and they affect the barriers between the planes.

Description
When Keyleth used Commune with Nature, she sensed the ley lines as a fuzzy, blue, particulate or mist-like energy that curled up in several lines and split, web-like, in various directions as they traveled beyond the horizon. They travel around the world and form a network of magical pathways. Later, when Keyleth and Allura cooperated in a Scrying ritual, those present could see tiny, sparkling, white-and-blue ley lines curled in circles and patterns around the table.

The ley lines move and change size based on the time of year and celestial alignments. For example, on Winter's Crest each year, the ley lines across Exandria swell and cross, causing the barriers between the planes to grow thin. The recurring celestial solstice is a notably more powerful merging of ley energies, during which the barriers between the planes are thinnest; it is associated with some major historical traumas.

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: she had figured out how to twist and tie the ley lines running over Rootgarden Marsh, causing arcane magic to stop functioning within a 100-mile radius.

History
During the Age of Arcanum, siphons were placed at nexuses of ley lines across Exandria to gather raw energy for the crafting of powerful artifacts, including the Vestiges of Divergence. During that era, Vecna harnessed the celestial solstice to attack his foes at a whim from the Shadowfell and withdraw before resistance could be mustered, but eventually his enemies discovered his use of the solstice and reverse-engineered his methods, allowing them to attack his home base of Thar Amphala. Though all but two fragments of Vecna were destroyed, he had left instructions with his most devoted followers that would allow for his rebirth.

The terrible arcane energies released in the Calamity totally reshaped the flow of ley energy around Exandria. One of the climactic moments of the war was the fight to banish Tharizdun; Ioun baited the mad god to her central temple, Tharizdun nearly destroyed her, and her temple sank beneath the earth in her sorrow, but Pelor returned there after defeating Tharizdun and planted the Sun Tree because this location was the nexus of several ley lines. The Verdant Expanse was born from a dramatic flourishing of life caused by the shifting of ley energies into the area, causing overgrowth and uncontrolled magical instability that stranded many fey.

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 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. But weeks before Winter's Crest, Vox Machina led a rebellion against Briarwood rule and came to kill Delilah, so she performed the ritual early. She did not know it at the time, but the ritual was a success despite the timing being less than ideal.

The Remnants created another siphon at the location of another ziggurat, beneath the strange Smouldercrown Mountains in Marquet's Rumedam Desert. When Vox Machina traveled to Thar Amphala in early 812 PD, they could see the energy flowing in a faintly visible bluish-purple strand from each siphon to the top of Vecna's dark tower Entropis, where the collected energy let off a bright, pulsing, purple glow. Soon afterward, the power was used in a deadly ritual that brought back Vecna.

Planerider Ryn is an expert on arcane ley lines who, by 836 PD, forecast a possible shift in the veil between the planes, somewhat analogous to the sudden flipping of the Earth's magnetic field, that could be related to a cyclical anomaly of magical interference.

Trivia

 * 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 a "Raven Tree" there, hopefully along the ley lines of Exandria.
 * 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 atop the tallest tower of his estate that keeps track of the shifting of ley lines.