Wisdom does not mean infallibility.
The lore of the Diablo franchise has gotten deeper and more involved with every new game, expanding from the small town of Tristram and the catacombs beneath the cathedral there to a globe-spanning adventure that led to Hell itself and finally in Diablo 3 we went to the Heavens and Pandemonium in turn. We've seen nightmarish monsters, Baal's army overwhelm the Barbarians and the destruction of the Worldstone, the siege of the Crystal Arch and the Nephalem stand triumphant against a former Archangel with the power of all seven Evils in its grasp.
Despite the presence of Uldyssian and his followers, it was merely a chapter, a small one in a saga that links us from the creation of the world of Sanctuary to the present day. One cannot discuss the Sin War without discussing nephalem, for in a way, the nephalem are far more important to the cosmos than Sanctuary, Inarius or Lilith. More important than the Archangels or Prime Evils.
Because I promised you some Diablo Know Your Lores, today we're looking at the origins of the world of Sanctuary, the mortal realm that has seen the machinations of the Prime Evils as well as near-extinction at the hands of Malthael, the former Archangel of Wisdom and current Angel of Death.