Archbishop Lazarus was an adviser of King Leoric of Khandaras in the back story to the computer game Diablo. Lazarus was corrupted by Diablo's mental influence while the demon was still trapped within one of the soulstones used to contain Diablo and his brothers by the Horadrim. Diablo used Lazarus as a tool to aid him in his breaking of Leoric's will. Diablo and Lazarus' influence slowly weakened the sanity of King Leoric, causing him to believe that his own advisers (except Lazarus) were plotting against him and the neighbouring kingdom of Westmarch (Leoric's home land) was planning to invade Khandaras. Leoric ordered his armies north to preemptively strike against Westmarch. By this point, Leoric trusted only Lazarus.
While the king's armies were fighting a losing battle, Diablo decided that the corruption of King Leoric was taking too long. Diablo had Lazarus kidnap Leoric's son, Albrecht, and bring the child before the soulstone in the depths of the Horadric dungeons below the town of Tristram. Once there, Diablo corrupted the much weaker will of Albrecht and, with the soulstone placed within Albrecht's forehead, morphed the child's body into his own and began to summon demons and fortify the dungeon.
Believing the kidnapping of his son was an act of protest against the war by the residents of Tristram, Leoric began terrorising the town in an effort to have his son returned to him. When the defeated troops of Khandaras returned, they fought, killed, and were cursed by the mad king. Not long after the king's death, Lazarus gathered together the townspeople of Tristram in a frenzied rage and convinced them to venture into the dungeon to fight evil (demons had begun to terrorise the town after the king was slain), rescue the king's son, etc. The townspeople were led into a trap where they were ambushed by a large group of demons led by The Butcher. Few survived (and those that did were seriously wounded) and Lazarus fled deep within the dungeon.
Within the game itself, Lazarus is one of the final unique enemies the player must defeat. Guarded by powerful succubi and magi on the dungeon level just above Diablo, the player comes upon Lazarus just after he sacrifices a child (which isn't Albrecht). Before attacking, he tells the player (with his voice echoing thanks to some nifty sound effects) "You are too late to save the child. Now you will join him... in Hell."
Lazarus can teleport around and fire nasty bad energies out of his staff. Lazarus himself never seemed that hard to defeat to me, though the hellspawn and corrupted magi around him are quite a pain to deal with since to reach Lazarus involves walking into an area surrounded by those enemies.