Original concept: an evil cultists' fortress in Mungoth, the third layer of Gehenna. The three 'spires' conform to the Planescape rule-of-three, and the cultists have three different sub-groups, each vying for supremacy over the other two. The entire fortress is carved from living rock, and conveniently situated upon a small volcano. Beneath the spires a small town has flourished; it is hard to find shelter in Mungoth, and the inkeepers charge accordingly for their 'hospitality'. Due to its secluded location, the Cathedral doesn't see many pilgrims. Many that survive the perilous journey through ash and snow are seized and thrown into the sacrificial pit, so you can see it takes a special type of pilgrim.
It can also function as a generic evil-aligned city, or the Big Baddie's HQ, or whatever.
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Illustration by Domigorgon
