Shadow Weave

The Shadow Weave was a force of magic similar to the Weave, said to lie in between the strands of the Weave.

"The Dark Lady's Weave is the only true source of power. Servants of Mystra are restricted by their goddess, for Mystra has established limitations and the bounds of magic. Followers of the Dark Lady are subject to no such limitations."

- Oruana Drearon

History
The Shadow Weave originated from Shar rather than from Mystryl; Shar studied the Weave in ancient times and created her own dark distorted version. Though the Shadow Weave had existed for quite some time (at least several millennia), only around 1372 DR did it become known and widely used.

Though Shar sought to use Mystra's death to replace the Weave with the Shadow Weave, she had miscalculated; just as the Weave collapsed with the death of Mystra, so too did the Shadow Weave.

Using the Shadow Weave
A spellcaster who wanted to manipulate the Shadow Weave had to either worship Shar, permanently lose a piece of his or her mind, or make some sort of arrangement with Shar and his or her existing patron deity. A shadow adept was a spellcaster who specialized in using the Shadow Weave.

The Shadow Weave was best suited for dark magic that drained life or muddled the mind and the senses, but less suited for magic that manipulated energy or matter. Spells from the schools of enchantment, illusion, and necromancy were enhanced, while those from the schools of evocation and transmutation were impaired. The Shadow Weave was also superior for fueling spells that created darkness, while it was impossible to use it to create light of any kind.

Since the Shadow Weave was a separate force of magic, it was not affected by areas of dead magic or wild magic within the Weave. An antimagic field however countered all magic within its area, and that included Shadow Weave magic. Shadow Weave users had to also overcome any spell resistance a creature might have. It was harder for a Weave user to perceive, counter, or dispel spells created by using the Shadow Weave, but at the same time it was also harder for a Shadow Weave user to affect spells created using the Weave. Any magic item created using the Shadow Weave was a Shadow Weave item and was affected by the same benefits and limitations as a Shadow Weave spell.

Practitioners
There were a large number of wielders of shadow magic in south Faerûn around the time of 1372 DR. Although none would admit to skill in the art, there were rumors of a growing shadow cult within the Council of Elders of Halruaa, and that they planned on attempting to overthrow the followers of Mystra and Azuth. Further rumblings in the area had the Conclave of Halruaa on the verge of exposing and bringing this group to justice, while others had the two organizations in league with one another.

As Vhaeraun was the drow patron of shadow magic, he worked together with Shar in projects that concerned this magic. One was the School of the Shadow Weave, which had followers of both Vhaeraun and Shar among them.