"Today's humiliation will be your death knell one day!"
The alien energy Slaanesh used to compile the Soul Book came from the profane aura Medivh had left in her soul.
She took her shame and turned it into motivation, using it to spur herself on.
Slaanesh used Medivh's profane aura to compile the Soul Book of Allure, but she also had an ulterior motive.
The Library of Allure belonging to Goddess of Blood Moon Sunada stored Soul Books that all featured Medivh as the male lead.
Using the same Soul Books as weapons would make it much easier for her to corrupt those goddesses.
Then, taking over Goddess of Blood Moon Sunada's Library of Allure would be a breeze.
Goddess of Spores Luna was one of the core heroines of the Library of Allure, while Medivh was its sole male lead.
Her third battle against Goddess of Blood Moon Sunada was about more than just the two of them.
If she won, both Holy Remains Sovereign Medivh and Spore Overlord Luna would become her slaves.
If she won, she would be the greatest contributor to the Primal Abyss's victory in the War of Light and Dark, and the Seven Deadly Sins would look at her in a new light.
Besides, some things had already happened. Slaanesh hadn't even been wearing Goddess of Light Athena's face, so she didn't have to worry about Athena of Pride coming after her.
Slaanesh immersed herself in building the Library of Allure, reliving her humiliating experience from a while back again and again, searching for inspiration as she compiled new Soul Books.
At first, the Library of Allure held only Soul Books with Slaanesh as the heroine.
As Slaanesh's understanding of the process deepened, Soul Books featuring the likes of Goddess of Blood Moon Sunada