He was the former leader of the Church of the Holy Light and later on he was revealed as the Twilight Prophet of the Twilight’s Hammer cult (This was after the death of Cho’gall). He was high-ranking lieutenant of Deathwing and the Old Gods, and aided them in their efforts to destroy all life on Azeroth.
When he was younger he was a student of Lordaeron’s religious leader, Archbishop Alonsus Faol. He spent many years learning from his devoted master and helped the Church of Light construct its most striking monument, the Cathedral in Stormwind. He was slain at Wyrmrest Temple by Thrall and his allies.
He was the head of Holy Order of Northshire Clerics prior to the First War between humans and orcs.
According to some sources he fled with surviving citizens of Stormwind to Lordaeron. Other sources tell that he was already in Lordaeron during the First War and only learned about the destruction of Northshire Abey when he spoke with Lord Lothar. He met Uther while he was just a bishop.
In warcraft novels they’ve wrote that Alonsus was deeply troubled by the loss of the abbey and by the terrible attrition his clerics suffered in that conflict. Aided by his apprentice, Uther Lightbringer, Faol created the Knights of the Silver Hand and equipped the followers of the Holy Light with the armaments of war.
Alonsus Chapel, in Stratholme, was the site where he nominated Uther as the first of the paladins.