Shadow Weaver was once a student of the Etherian sorcerer Norwyn, with fellow student Castaspella. She showed a lot of promise before Hordak arrived on Etheria.
On Hordak's arrival he immediately looked for people he could bend to his will to conquer his new world. One of the first was Shadow Weaver. He promised her vast power for her aid, so much power that it was impossible for her to resist. He presented her with a Dark Gem that she could absorb the power of. Norwyn became of aware of Shadow Weaver's betrayal, and destroyed the gem when she only absorbed about a third of it's power. The gem's destruction caused not only physical damage, but also scarred her mentally and spiritually.
Over time Shadow Weaver has become Hordak's second in command, as well as his adviser. Although Shadow Weaver feels no loyalty to him, and has betrayed him on numerous occasions. On one of those occasions she summoned Hordak's old student Skeletor, a plan that might of worked if Skeletor didn't seek Hordak's throne for himself.