Also initially we wanted to have a "v3 orc slums invasion" style mechanic behind mythalite, where good guys could deposit it in a machine at the HC Estate to "refuel" Arabels magical shield and prevent the broken seal from leaking fiends into the city (Kanthea was MIA at start of v5). The shield would weaken over time.
At the same time bad guys could use the incantum malefacarum ritual to deposit mythalite to damage the city defences
When weak enough demons and devils would leak into old town and battle PCs and each other. If really weakened some could even enter the citadel.
The mechanic was never implemented by code though a few DM run events played out the same mechanic and Kanthea was eventually found (I think puffy and hominid ran most of this? I believe the plot went something like "Kanthea and a sun elven house in a fortress in avernus trying to stem the tide ever since the seal she maintained was shattered after the primordial attack")