Tathamet

Tathamet is the twin sister of the being known as God and the older sister of Life and Death. Tathamet is also the adoptive mother of the Archangels. Tathamet is one of the two supremely powerful primordials alongside her brother, God. She is the complete antithesis of her brother. Whereas her brother embodies Light and Existence, Tathamet embodies Darkness and Nothingness. Tathamet was sealed away long ago before Creation began in a terrible war between her brother, Life, Death, and the Archangels. Her name is now whispered in fear by the older angels in Heaven and the denizens of Hell and Earth regard her as nothing more than a bedtime story.

Tathamet was unsealed when the Mark was broken by the combined efforts of Grauzauberer and DxD.

History
Before Existence, there was Nothing and Nothing was all there was. At some point in time, Being came to be marking the beginning of all existence. Existence began to create entire worlds and Nothingness became jealous. Fearing that these lesser beings would take her brother from her, Nothingness destroyed them. From the interplay of Creation and Destruction was born Life and Death. At some point in time, the seven Archangels were created and they simply were. Existence created one more world specifically for Nothingness and she destroyed it. With Existence's patience gone, he and his children and siblings waged a terrible war against Nothingness with entire universes being created and destroyed in an instant.

After countless eons, Nothingness, who would be later known as Tathamet, was sealed away with her eldest, Helel, bearing the lock and key to their cage. However, she, with the last of her strength, implanted a small portion of her consciousness into the Mark. She would corrupt Helel and cause him to fall to Earth. Helel, who is now known as Lucifer, would later pass the Mark to Cain.

Thousands of years later, Cain passed this Mark onto Issei Hyoudou. Issei fell to the influence of the Mark. During his battle with Irina and Vali, the Mark would be destroyed by the combined efforts of DxD and Grauzauberer and she would be released into the world once more.

Appearance
Upon her release, Tathamet manifested a human form to Issei. This became her normal human form. Her human form takes the appearance of a young woman of inhuman, unfathomable, and unparalleled beauty in her teens wearing a dark clothing. She has hair as dark as dark as the void and eyes that exude power itself. Tathamet also wears a red scarf as well as red hair clippings.She is said to be even more beautiful than Gabriel the most beautiful angel in Creation and her daughter.

While she recovered her power, Tathamet possessed Ravel Phenex taking on her appearance.

Her true form is said to be incomprehensible. It is nornally perceived by others as a mass of absolute darkness that extends infinitely in all directions. When released, Tathamet took on the form of an expanding field of black smoke. During the Second War, Tathamet took the form of a massive dragon with seven heads.

Trivia

 * Tathamet is based off of Supernatural's The Darkness, Chaos King and Oblivion from Marvel, and Tathamet from Diablo.
 * Her appearance is based off of Ayano Tateyama and Azami from Kagerou Project and Tathamet from Diablo.
 * Tathamet's relationship with God is similar to the Chinese concept of Yin and Yang. Her often antagonistic relationship with her brother can also be likened to the Zoroastrina deities, Ahura Mazda and Angra Mainyu, the Gods of Light and Darkness.