Ashfall Cathedral

When the forge below first lit, the column of ash it threw up came down for eleven days. It buried a cathedral whole: three metres of warm grey ash over the roof, and the doors sealed from outside by the weight of it. The congregation did not leave. They had been mid-service, and they are still mid-service, and they have been for long enough that the service is the only thing any of them remembers how to do.
The ash is warm. It has always been warm, and they have decided that this is grace. They sing to a god assembled out of cooled slag, a thing on the altar with no face that occasionally shifts, and their liturgy has drifted over the decades into something that is mostly about heat. They are not hostile on principle. They are hostile because you are letting the cold in, and from the inside that is a reasonable objection.
Encounters

Tier 1
Ash-Choked Penitent

Tier 2
Cinder Deacon

Tier 3
Kneeling Censer

Tier 4
Pyroclast Templar

Tier 5
Bishop Ossuar (Mini-Boss)

Tier 6
Slag Confessor

Tier 7
Ashfall Inquisitor

Tier 8
Reliquary Colossus

Tier 9
Voice of the Buried Choir

Tier 10
Saint Kalcyne of the Grey Communion (Final Boss)
The final boss
Saint Kalcyne of the Grey Communion was the officiating priest. He has not stopped speaking the same sermon since the first day and he is now largely made of the ash he has been breathing. Bishop Ossuar handles the doctrine; the Voice of the Buried Choir handles the singing. Kalcyne handles only the sermon, and he will finish it, and the fight is what happens while he does.
Before you enter
The recommended level is 55. Treat it as a starting point: farm earlier tiers, upgrade gear, and adjust your pet when a boss stops you.
Difficulty inside a dungeon comes from one number that climbs with the tier, and the step between tiers is a large fraction of the gap between dungeons. So a wall at tier 7 here is not a signal to grind this dungeon harder - it usually means the opening tiers of the next dungeon are a better use of the hour, and you come back with the gear they dropped.
