Archbishop Lazarus was the advisor of king Leoric during his reign of Khanduras and a high-ranking official in the Zakarum faith. It was he who released Diablo from his soulstone which allowed the Prime Evil to begin corrupting Leoric, thus setting in motion the events of Diablo I.
Though he does not appear directly in Diablo III his legacy remains and his actions provide the foundation for much of the beginning of the game.
As one of the archbishops of Zakarum Lazarus was a high-ranking official in the church, likely just below the High Council itself. When he left Kurast the leadership of the Zakarum was already under the influence of Mephisto, and Lazarus was likely tasked with finding Diablo's soulstone and releasing the Prime Evil. He pressured Leoric to choose Tristram as his seat of power for specifically that purpose, and he quickly ventured down into the labyrinth beneath the Cathedral to release Diablo after arriving.
After releasing Diablo, Lazarus returned to the surface and did everything in his power to aid Diablo in possessing the king. Though this proved rather uneventful during the first years, but the fifth year the king was growing increasingly mad under Diablo's influence. His other advisors, unaware of Lazarus demonic allegiance and of the king's growing corruption, began to grow more and more suspicious of their king's actions. Lachdanan in particular, the captain of Leoric's knights and an honorable man, proved to be a problem. Lazarus repeatedly admonished Lachdanan for doubting his king, but his insistence proved to be problematic. In a desperate attempt to discredit Lachdanan, he tried to accuse him of treason against Leoric. The circumstances of the accusations are unknown, but the rest of the court rejected the accusal outright, for no one believed Lachdanan would ever betray his king.
Fearing that Lachdanan and other inquiring advisors would eventually cast too much doubt on Leoric, he convinced the king that Westmarch was preparing to attack them. Leoric promptly declared war on Westmarch, and though his advisors urged him not to, it forced many troublesome individuals from Tristram and far away, including Lachdanan who led the army.
Though Lazarus's plan has succeeded, Diablo's did not. When Diablo tried to fully subjugate the king's mind he still encountered resistance. Realizing he could never fully subdue the king, Diablo abandoned Leoric and ordered Lazarus to bring him prince Albrecht, Leoric's son, to him instead. Lazarus abducted the boy and brought him down into the labyrinth.
By the time Lazarus returned, Leoric has already been slain by Lachdanan and the townfolk were both fightened and without leadership. He convinced them that he had been possessed by the same darkness that had taken Leoric, but that he had broken free of it. Reminding them that Albrecht was still down in the church, they rallied the townsfolk into a mob and led them down into the Cathedral where the The Butcher and his demons lay in ambush for them. As the townsfolk were slaughtered, Lazarus once more raced down to the very depths of the labyrinth.
He was finally slain when Aidan found him on his way to Diablo and slew him in order to get access to the deepest level of the monastery.