The practice of crafting and casting arcane magicks is known across Faerûn as the "Art"; the threads of magic from which a mage's spells are drawn is called the "Weave". The "Shadow Weave" is an alternate force of magic that is the inverse and opposite of the Weave. The Shadow Weave does not originate from Mystra, the goddess of magic, but from the dark goddess Shar.
