Enter the Tristram Cathedral northeast of Adria's Hut. Your objective, in the Legacy of Cain quest, will now be to enter Cathedral Level 1. The entrance to the first level of the Cathedral is at the end of the corridor, fight your way through the Risen and other undead that are blocking the hallway, then descend into the Cathedral level 1. There will be a checkpoint as you enter the level, in which you will respawn in case you die. Your goal now is to search for signs of Deckard Cain in the Cathedral.
"The spirits of the cathedral tell me of a great evil that walks these crumbling halls once again. Dark memories return to torment them. Could Cain have possibly survived such danger?"
Go down the stairs, then towards the blue pit, and turn right, going away from the pit. There are some monsters blocking the way. Note the Chandelier Chain on the side. You can interact with it to have a Chandelier fall on them, either killing them or wounding them badly. Or you could just fight them. The mobs here include Risen and Walking Corpses.
Break the wooden door and go through it. There are plenty of barrels, weapon racks, and other sources of loot in this area. Some of the elements of the dungeons in Diablo 3 are random, so treasure location in one playthrough might be different then the one in another playthrough. Dungeon layout is also randomized, so we will give you detailed descriptions of the dungeon environment and not step by step directions. The important thing to note, is the name of the exit that takes you to the next phase of the dungeon/quest. We will generally show you screenshots of the destination, so that you can identify it, no matter what dungeon layout you got.
Enter a chamber with a big circle in the center. Her you will encounter new monster types, such as Carrion Bats, Grotesques, Ravenous Dead, and Corpse Worms. Other chambers, include loose stones which you can search for some loot, and wooden barricades blocking your path, which you can break. A Scribe Lectern in one of the corridor's also has more lootable items. There are also shrines, such as a Desecrated Frenzied Shrine, which you can restore to get +25% attack speed and crit bonus.
The exit you are after is called "LEORIC'S PASSAGE".
HERE IS WHAT YOUR EXIT LOOKS LIKE:
When you get close to it, a blinking yellow circle appears on your radar. Head towards the circle. Leoric's Passage is a big arch between two big statues, with light coming out of the passage. Enter Leoric's Passage to advance to the next stage.