The Archangel Malthael is a member of the Angiris Council. He is the solemn and mysterious Archangel of Wisdom, and is speculated to also somehow hold the aspect of Death. Mathael is closely associated with Chalad'ar, the Chalice of Wisdom, but wields a pair of sickle swords in combat. He served as the guiding hand and first among equals on the Angiris Council.
Malthael is a gaunt figure among the Angiris Council. He wears black robes and a black breastplate.
As the Angel of Wisdom Malthael holds many ancient and powerful relics and artifacts, including the Worldstone, which was stolen by Inarius and used to create Sanctuary, and was long agonized over its loss.
When Sanctuary was discovered, he abstained from the vote to decide its fate, claiming that at the end of all things only he will prevail, despite this the rest of the Council(or at least Imperius) counted his abstinance as a vote for the destruction of Sanctuary.
Malthael was unable to comprehend the destruction of the Worldstone and left the Angiris Council. Tyrael says that Malthael has gone mad, searching for the unknowable truths of life and death, endlessly wandering Pandemonium. Since he did not die, no new arch-angel has formed from the Crystal Arch to replace him, leaving a perpetual gap and imbalance in the Angiris Council. Following the rebirth of Diablo as the Prime Evil, and the Lord of Hell's destruction at the hands of the new Heroes, Tyrael took Malthael's place on the Angiris Council as the Angel of Wisdom. (It is worth noting here that Tyrael claims he will be amoung them as a mortal leading to the idea that Malthael is still possibly alive somewhere.)
Malthael has yet to appear in any Diablo series game.