The Unholy Altar is a church-style sub-level, which is accessible after the Archbishop Lazarus quest is initiated. Once the quest has been given to the player from Deckard Cain, a Red Portal will appear on the large pentagram (located somewhere in Level 15) and allow you to enter the Altar.
Like the rest of the quest-related sub-levels in the game, the Unholy Altar is exclusive to single player, and has a pre-defined map which the player must explore. The area is riddled with monsters - specifically, high-level Succubi-variants and/or Advocates. Some parts of the level are gated off and sealed to begin with. On either side of the level, there are pedestals holding the "Book of Lazarus" - which, when activated, will teleport the player to one of the now-open areas of the level, which also reveal themselves to contain more monsters.
After both of the secret areas are opened, the player must return to the room where they first appeared in the level and walk on the pattern in the ground - triggering a teleport to the previously closed-of center section, and initiating a cutscene of the now corrupt Archbishop Lazarus in Tristram Cathedral.
- "Abandon your foolish quest! All that awaits you is the wrath of my master. You are too late to save the child. Now you will join him...in Hell!"
- Archbishop Lazarus, upon the player's arrival.
The player will then have to fight the mad Archbishop, two unique Hell Spawn (Red Vex and Blackjade), and a group of other monsters. The Red Portal will reopen upon the death of Lazarus, and transport you back to Level 15.
There is no way out of the level other than the Red Portal or casting Town Portal. After completing this quest, the way will be clear to receive the Diablo quest (which is also given by Cain) and venture into Level 16, by entering the now-illuminated pentagram.