Inarius was an Archangel of the High Heavens, once part of the Angiris Council, and one of the creators of Sanctuary.
The story was retconned after the first manual was printed for Diablo 1 (see below)
The Sin War Background
Inarius together with other rogue angels and demons created Sanctuary and the Nephalem (angel and demon hybrids with great power). However the development of the Nephalem's powers infuriated Inarius, making him want to start anew by eradicating all the Nephalem. However this brought him into a conflict with Mephisto's daughter (and his romantic partner) Lilith. This conflict resulted in the banishment of Lilith and the creation of the Worldstone.
Inarius used the worldstone to prevent the Nephalem from developing their powers, reducing each succeeding generation's power until they became normal humans of Sanctuary. Inarius also used the worldstone to hide the existence of Sanctuary from the High Heavens.
Inarius later created the religion Cathedral of Light to make the beings of Sanctuary worship and serve him. Inarius is also know as the Prophet of the Cathedral of Light. He's also a former member of the Angiris Council. Inarius has a son with Lilith named Rathma whom is the predecessor of Necromancers and a follower of balance. Rathma works closley with Trang-Oul to keep the balance of Sanctuary.
Inarius was punished by the Angiris council and was given as a gift to Mephisto for his wrongdoings.
Diablo 1 Background
This was formerly the background for Inarius, found in the Diablo I Manual, under the Overlords section:
- Angel Inarius, was proud of his beauty and boasted loudly of his purity and worth. His thoughts became so clouded that he believed himself to be above both Angel and Demon, and he left the High Heavens to form his own dominion. He constructed a great cathedral of mirrored glass and crystal, and followers flocked to him, drawn by his numinous charm and wealth. Once Inarius had gathered a seizable army, he decided to prove his power.
- Inarius first laid siege to an infernal temple devoted to the worship of Mephisto, but made the foolish mistake of overestimating his prowess. The armies of Inarius laid waste to the temple and killed the dark monks that dwelled within. The Three Evils had considered the vain warrior a mere nuisance and amusement until this time, but this was an insult that they could not bear.
- Mephisto himself is said to have appeared at the cathedral of Inarius. He laid waste to the church and the surrounding countryside. The Lord of Hatred took the proud archangel and his followers captive. He bound Inarius with tremendous chains and slowly tore the wings from the back of the angel. Great barbed hooks were then used to stretch out the once glowing skin and his features were distorted by vile powers. Many of the followers of Inarius were given as gifts to Baal and Diablo, but the rest were molded to match the bloated image of the now crippled angel. To this day, Inarius is said to be trapped in Hell within a chamber of mirrors, his eyelids torn from his face as he is forced to gaze upon his misshapen form for all eternity. His misguided followers now serve as Hell's taskmasters, taking the anguish of their lost glory out upon the bodies of others.