Ley

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

Description
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, although she could not sense them beyond several natural boundaries.

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. The movements of ley lines can be tracked and measured using a device called a geometer, which resembles a weather vane consisting of a series of spinning concentric rings.

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.

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 leylines 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.

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.

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.