Vischar Orous is the Chief Librarian and Zharesh Covenant of the Brotherhood of the Vizjerei. As such he is a highly knowledgeable and often referenced indivdual.
Not much is known about him, personally, though it has been speculated that he is a figure not unlike Abd al-Hazir.
His writings appear in the Diablo I Manual, where he writes most of the beastiary section. Some of the things written in that, however, have been retconned out of existence. For example, he called the Goat Men (now called Khazra) demons, which in the lore is now false. This implies he either believed false information, or (in this case) covered up the corruption of the Khazra by his own Vizjerei brotherhood centuries earlier.
His status at the time of Diablo III, much like his entire life, is largely unknown. He may well appear in Diablo III, though his absence from Diablo II seems to make that unlikely.