Mirakodus was the name of the archangel in the visions of Lord Juris Khan. This archangel asked him to gather his priests and mages to cast a complex spellwork that would allow Ureh to transcend to Heaven in order to escape Diablo and his minions who surrounded the city. (page 172)
The tall angelic form glowed as bright as a sun. Clad in glorious bright platinum armor, intricate runes and sculptured glyphs decorating hsi breastplate. His shoulders radiated a display of crackling, writhing tendrils of pure energy, fiery wings. He held a flaming sword.
His hood was thrown back, in contrast with other angels which are depicted as hooded faceless beings. Golden hair with a visage of perfection. Captain Kentril Dumon saw a magical image of Archangel Mirakodus made by Lord Juris Khan in Ureh. The beauty and perfection of the image of this archangel was rapturing. However, as the image drew his very soul, and engulfed his mind with horror, one name in all of Sanctuary came to mind that could inflict such terror within him ... Diablo.
Diablo disguised to fool Lord Juris Khan into casting a spell to create an inbalance in Sanctuary by transcending Ureh to Hell, but this failed when Gregus Mazi broke away from the ritual spellwork, sending Ureh to a timeless limbo between Heaven and Hell. More details over at Diablo: The Kingdom of Shadow.
Speculation
It could be interpreted that Diablo was formerly known as Archangel Mirakodus. If Diablo: Demonsbane holds any canonical value, according to Tyrael the secret his brethren so much guard is that the creatures of the Burning Hells were once angels.
References
- Diablo: The Kingdom of Shadow Barnes and Noble