Angelus

Angelus is the first of the angels, who named themselves after him. Along with Demonus he is the first being to exist since Anu and Tathamet died.

Biography
Shortly after Anu and Tathamet killed one another with their final attacks Angelus, along with his "sister" Demonicus, spawned from the bodies of the two beings.

Angelus and Demonicus were opposite beings by their natures, with Angelus being Spirit and Light while Demonicus was Physical and Darkness; despite this the two were at peace with one another and acted as siblings for a 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.

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

Soon afterwards the two new species waged war with one another, this conflict would soon cause Angelus and Demonus 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, Angelus and Demonus engaged one another in combat near 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 fought for months and the energy released wiped out their younger siblings many times only for them to return. Demonus eventually refused to fight anymore and Angelus saw his sibling split herself into seven different parts, these seven parts eventually becoming the Prime Evils.

Seeing this caused Angelus to thrown down his five weapons and leave the conflict in disgust.

Soon afterward the other angels and demons reformed, unaware of the previous conflicts. Five angels, Tyrael, Malthael, Auriel, Itherael, and Imperius, located these weapons and were infused with power by them, becoming the Angiris Council in the process.

Angelus searched himself deeply, finding the truth of his status as the Heart of Anu.

Once the Eternal Conflict started again Angelus remained hidden as the Guardian of the Afterlife, and when the Nephalem came to be Angelus arrived on their world and remained as a teacher.

Powers

 * Extremely Powerful Angelic Physiology: As the first being of light to come into existence and the Heart of Anu, Angelus is one of the strongest beings in all of creation with only his counterpart Demonus being able to match him in raw power and skill. Any powers that younger angels display are held by Angelus at much higher levels. Angelus is powerful enough that, if pressed, he could assault the Burning Hells alone and be victorious.
 * Immortality: Angelus, like all angels, is immortal and cannot die from old age. He and Demonus 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 Demonus or an equivalent being has the necessary raw power to kill Angelus.
 * Teleportation Like other angels, Angelus is capable of teleporting an unknown distance in a burst of light.
 * Weather Manipulation: Angelus is so powerful that when he confronted Demonus in Pandemonium storms of light brewed in the skies and fires of darkness erupted from the ground from the simple fact that the two beings were preparing to fight one another.
 * Light Manipulation: Angelus is capable of manipulating light for a variety of purposes with extraordinary power, far above any other angel's usage. Angelus notably used this power separate Leah from Diablo and send her back to Sanctuary shortly before facing the Prime Evil.
 * Possession: Uniquely among angels, Angelus can possess a human or Nephalem as a host. He usually requests their permission beforehand, but if absolutely necessary he will forcefully take it.
 * Supernatural Perception: Angelus could detect Diablo becoming an entity akin to Demonus from the afterlife, and could effortlessly see Diablo past his possession of Leah.
 * Weapons/Combat Master: The weapons held by the Angiris Council were originally Angelus', who had achieved mastery over them all in combat. When facing off against Diablo, who had Demonus' power, Angelus held the advantage due to sheer skill in combat. Diablo required outside assistance to overwhelm Angelus.

Vulnerabilities

 * Vast Quantities of Darkness/Powerful Objects infused with Darkness: An incredible amount of darkness is able to weaken Angelus, with objects such as the Body of Tathamet gradually sapping his strength and allowing weaker beings to injure and possibly defeat him.
 * Demonus: As his equal in power and a being of darkness, Demonus is able to match and possibly kill Angelus.