Demonus

Demonus is the first of the demons, who named themselves after her. Along with Angelus she is the first being to exist since Anu and Tathamet.

Biography
Shortly after Anu and Tathamet killed one another with their final attacks Demonus, along with her "brother" Angelus, spawned from the bodies of these two individuals.

Demonus and Angelus were opposite beings by their natures, with Demonus being Physical and Darkness while Angelus was Spiritual and Light; despite this the two were at peace with one another and acted as siblings for untold eons in what would become known as Pandemonium.

Eventually the Spine of Anu and Body of Tathamet forming the High Heavens and Burning Hells around these remnants, along with younger beings of light and darkness forming shortly thereafter.

The beings of darkness discovered Demonus and named their species after her while the beings of light did the same with Angelus.

Soon afterwards the two new species waged war with one another, this conflict would soon cause Demonus and Angelus to side with their younger "siblings", as despite their prior relationship neither could stand to see the beings so similar to themselves killed.

One day, after the conflict proved eternal due to the never-diminishing armies, Demonus and Angelus engaged one another in combat near the Eye of Creation or the Worldstone.

The power and emotion of the two caused the skies to cry and the lands to roar, devastating Pandemonium with power only comparable to Anu and Tathamet.

The two forces fought for months with their younger siblings being utterly wiped out an uncountable amount of times but always returned. Demonus eventually refused to fight any longer and in a desperate attempt to stop the fighting split herself into seven different beings.

These seven parts eventually became purely malicious demons called the "Great Evils", who continued the conflict and, along with the all angels and demons after Angelus made them forget about himself and Demonus, came to believe they were directly spawned from Tathamet's heads.

When one of the Great Evils, Diablo, used the Black Soulstone to command the essences of the other Evils he held a visage that was exactly the same as Demonus.

Powers

 * Extremely Powerful Demonic Physiology: As the first being of darkness to come into existence and the Heart of Tathamet, Demonus is one of the strongest beings in all of creation with only her counterpart Angelus being able to match her in raw power and skill. Any powers that younger demons display are held by Demonus at much higher levels. Diablo, after consuming the power of the other Great Evils, came to be just as strong as Demonus.
 * Immortality: Demonus, like all demons, is immortal and cannot die from old age. She and Angelus lived for untold billions of years with only each other until the Spine of Anu and Body of Tathamet caused the High Heavens, Burning Hells, and all their inhabitants to form. Only Angelus or an equivalent being has the necessary raw power to kill her. Demonus was also able to kill herself in a manner, which resulted in the seven Great Evils coming into existence from her body.
 * Teleportation Like other demons, Demonus is capable of teleporting an unknown distance in a burst of dark fire.
 * Weather Manipulation: Demonus is so powerful that when she confronted Angelus in what would one day be known as Pandemonium storms of light brewed in the skies and fires of darkness erupted from the ground simply from the two beings preparing to fight one another.
 * Darkness Manipulation: Demonus is capable of manipulating darkness for a variety of purposes with extraordinary power, far above any other demon's usage.
 * Possession: Diablo demonstrated this power after absorbing the other Prime Evils, Demonus would've been capable of the same feat.

Vulnerabilities

 * Vast Quantities of Light/Powerful Objects infused with Light: An incredible amount of light is able to weaken Demonus, with objects such as the Spine of Anu gradually sapping her strength and allowing weaker beings to injure and possibly defeat her.
 * Angelus: As her equal in power and a being of light, Angelus is able to match and possibly kill Demonus.