In the Forgotten Realms campaign setting for the Dungeons & Dragons role-playing game, the Shadow Weave is a force of magic that is the inverse and opposite of the Weave. The Shadow Weave does not originate from Mystra, but from the dark goddess Shar. A spellcaster who wants to manipulate the Shadow Weave must either worship Shar, or permanently lose a piece of his or her mind. A shadow adept is a spellcaster who specializes in using the Shadow Weave.
The Shadow Weave is best suited for dark magic that drain life or muddle the mind and the senses, but less suited for magic that manipulate energy or matter. Spells from the schools of Enchantment, Illusion and Necromancy are enhanced, while those from the schools of Evocation and Transmutation are impaired. The Shadow Weave is also superior for fueling spells that create darkness (spells with the darkness descriptor), while it is impossible to use it to create light of any kind (spells with the light descriptor).
Since the Shadow Weave is a separate force of magic, it is not affected by areas of dead magic or wild magic within the Weave. It is harder for a Weave user to perceive, counter or dispel spells created by using the Shadow Weave, but at the same time it is also harder for a Shadow Weave user to affect spells created by using the Weave. Any magic item created using the Shadow Weave is a Shadow Weave item and affected by the same benefits and limitations as a Shadow Weave spell.
In the computer game Neverwinter Nights 2 the Guardian of Illefarn was supposed to be bonded to the Weave. Because Netheril nearly destroyed the Weave with their attempt to ascend to godhood, he was forced to turn to the Shadow Weave, and became a creature of darkness, destroying Illefarn.