|Spoiler Alert: This article or section contains details about plotlines not covered in the anime. Do not proceed unless you wish to be spoiled.|
Ren Ashdoll is the Darkness Elemental Lord that is been buried in forgetting dream of Human Realm. She is also the one who gave the "will" about fighting the 5 Elemental Lords that have been corrupted by otherworldly darkness.
After the blade dance 3 years ago and the first assassination of the Elemental Lords, it was hinted that Ren Ashdoll's "will" has been corrupted.
Ren Ashdoll looks similar to Restia.
For unknown reasons she once waged war with the other five Elemental Lords, a war that involved many powerful spirits like Restia, Scarlet and Bahamut. What provoked this is unknown, and it's not known whether she did it for selfish or upstanding reasons, or whether the other lords had already been corrupted or not, however it is known that her will in Restia was not corrupted until 3 years before the story begins, meaning it's likely she was perfectly sane at the time.
It is revealed that the Ren Ashdoll took an interest in humans. Ren Ashdoll sensed some kind of potential in mankind. Hence, on the brink of demise, she entrusted a portion of her power to them, instead of the powerful races of the dragons and giants. Her prediction came true as human maidens managed to become elementalists via the mean of a Spirit Contract.
Whether it's because of the "Otherworldly Darkness" influencing her within Restia or just her own nature, she seems to want Kamito to embrace his malice and become a tyrannical Demon King, and she has no reservations towards what he has to say in the matter, enforcing her will upon him when he resists.
Restia herself being a representative of her will, and openly trying to stop her, implies this is not Ren's usual mental state, however there's also the possibility that Restia is simply rebelling to defend Kamito.
Ren Ashdoll is one of the Elemental Lords. For unknown reasons, a war broke out between Ren Ashdoll and the other five Elemental Lords. This was called the Spirit War, which took place several millennia ago. After her defeat in the Spirit War, Ren Ashdoll's powers were reincarnated in the human realm within male hosts, those who were known as Demon Kings, most notably Solomon who lived a thousand years ago. Kamito is the current reincarnation of Ren Ashdoll's Darkness Elemental Lord powers. On the other hand, Ren Ashdoll's will and consciousness resided within Restia until the most recent Blade Dance. It is currently unknown whether Ren Ashdoll's will continues to remain inside Restia after her purification by the Demon Slayer. It was revealed that the Darkness Elemental Lord created Restia using the Otherworldly Darkness. She also entrusted Restia with a Black Box containing a vast quantity of memories that was manifested from Restia's consciousness with her knowledge.
Enchanted/ Rebirth Magic - The Darkness Elemental Lord used a spell on Restia to temporary sealed away her Spirit attributes. This probably belongs to the same type of magic as what she used to transfer her powers to the Demon Kings.
Spirit Chants Edit
—O Demon King, go on and release the true darkness upon this world.