The Tristram Cathedral is an ancient Horadrim edifice. Under it, they imprisoned Diablo in his soulstone. When the Horadrim faded away, the cathedral was left as decrepit ruin. The cathedral briefly became the seat of power of King Leoric, but with the The Darkening of Tristram and the town's destruction, the cathedral has since remained abandoned.
"We have just arrived in Tristram, and I must say I'm a bit dismayed. This place is a backwater filled with serfs and an ancient, broken-down monastery, hardly fit for the king of Khanduras! I cannot fathom why Lazarus was so intent on this becoming our new seat of power."— Leoric in his journal(src)
After the Horadrim defeated Diablo, they buried his soulstone in a secluded cave near the River Talsande. Above this cave the Horadrim constructed a great monastery from which they could continue to safeguard the soulstone. As ages passed, the Horadrim constructed a network of catacombs beneath the monastery to house the earthly remains of the martyrs of their order, and also used it to store the weapons they had used during their war against the Prime Evils. As time passed however, the Horadrim fell into obscurity, and with their passing, the cathedral fell into ruin. Although villages grew and thrived around the shell of the old monastery, no one knew of the dark, secret passageways that stretched into the cold earth beneath it.
With the coming of Leoric and his Order of Light, the cathedral was appropriated as his seat of power, and was restored to its former glory. Multiple potions were stockpiled as the clergy used them to treat the sick and wounded.
The Darkening of TristramEdit
"When we got to Tristram, we took our time before we got down to the business of actually going into the cathedral. Town had a nice inn, if I recollect. Truth is, there was something evil coming out of that old church: you could feel it."— An adventurer(src)
Sensing his chance to escape, Diablo lured Archbishop Lazarus into the catacombs beneath the cathedral, and compelled him to shatter his soulstone. Thus began a series of events that would become known as the Darkening of Tristram. Leoric served as Diablo's original host, but stymied from fully controlling the king, Prince Albrecht was brought into the depths of the labyrinth to serve as Diablo's vessel. Albrecht's fears were made real, and large bodies comprised of living rock erupted from the walls and bowed to their dark master. The ancient, skeletal corpses of the Horadrim arose from archaic crypts and lumbered off into the red washed corridors beyond. Such was Albrecht's terror that Hell itself spilled over into the mortal realm, twisting the labyrinth beneath the cathedral into a bastion of terror.
Leoric died at the hands of Lachdanan, ending his reign of terror, but the people of Tristram couldn't rest easy. Strange, eerie lights shone out from the monastery, and strange creatures could be seen venturing out from the church. Lazarus led a group of townsfolk into the labyrinth in a bid to find Albrecht, but it was a trap, as they were set upon by a demon called the The Butcher. Only a few returned to the surface to recount what had transpired. A hellish drone emanated from the cathedral, and the incantations of dark rituals could be heard.
Dark rumors spread throughout Khanduras as to the evil that had taken hold of the cathedral. Warriors, Sorcerers, and Rogues alike were drawn to Tristram in response, whether it be for riches, or to discover Horadric tomes. A few warriors ventured into the cathedral, but were never heard from again.
In the years that followed Tristram's destruction, a collection of merchants looking to profit on adventurers and travelers drawn by legends of riches within the old cathedral came to settle near the cathedral, founding the town of "New Tristram." The cathedral was stripped bare of riches, and afterwards, New Tristram fell into decline. Notably, the cathedral was visited by Abd al-Hazir, but he could only muster the courage to take a few steps into the structure before departing.
The Fallen StarEdit
The cathedral does not appear in Diablo II, as the portion of Tristram where the cathedral would be is not accessible (see image).
Once the quest advances further and Leoric's crown is found, one may open the door to the second level.
The final level ends in access to the Royal Crypts, where you fight the Skeleton King and gain access to the Desolate Chamber where the crater of the Fallen Star and the stranger at its center can be found.
The Cathedral is mostly filled with Risen Dead, Hungry Corpses, Skeletons, Skeleton Archers, Grotesques and Skeletal Summoners, the latter of whom can appear from murals on the walls. Level 3, as mentioned above, is where the cultists and Jondar are.
It also features unique environmental traps: chandeliers. Each chandelier can be released by touching its supporting chain, falling on top of the area marked with an X-shaped shadow. Monsters below will take Fire damage and be Stunned. There is an achievement for killing a thousand monsters with it.
The Black Mushroom can be found in the Cathedral.
The cathedral as it appears in Diablo I becomes replayable in the Darkening of Tristram event.
The cathedral appears in the Blizzard World map in Overwatch.