Wisdom does not mean infallibility.
Before the coming of the Nephalem, Imperius was the unquestioned greatest warrior in all creation.
From this whirling nightmare, this bloated ruin of the first evil's corpse crawled the demons. The Burning Hells could in a real sense be said to be the blackened corpse of Tathemet itself, and at the center of the Hells lies the Black Abyss, from which rose the Great Evils. Essentially birthed from the seven heads of Tathemet, there were the three Primes -- Mephisto, Baal and Diablo -- and the four Lessers.
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.