Hell Lumia drops in and kicks the shit out of the potential Apocalypse
Oh ho,
rant time.
And this is because they allowed a dangerous, extradimensional entity run amuck on their world, defiling the bodies and destroying the minds of their followers whilst most of them do fuck all.
You can get Mallach IN A ROOM with Kassyra in her romance path, and he just gives her more or less a warning. This is the
single entity that is really posing a threat outside the wraiths to the north, which only Lumia seems to be concerned with, at least as far as the deities with a local presence go.
Interested in her work or not, what she's doing is so incomprehensibly bad for them, opening up portals to the void, carving holes out of their sphere of influence, realistically giving them all a bad name if the NPCS actually thought rationally and instilling doubt within them.
It would take one of the Seven to just vaporize Kassyra as she is now, and be done with it. They have bigger threats to deal with? Then just shoot the smaller one in the face before it grows so you can focus entirely on the bigger one, you do not want a threat on 2 fronts. You don't even need to put much effort into stopping the rest of the demons, kill Kassyra and only Tollus's flock will really remain a coherent force, with the rest forming isolated little debauchery bubbles that are easily stomped out by local forces and heros.
Hell, Keros is just down the road half the time, and didn't pop in to lend Lumia a hand with
an honest to god wraith incursion just down the street when Lumia has her little basement disaster to deal with. Keep in mind, a single wraith gaining sufficient power is something that threatens the entire world, let alone the status quo of the divine armchair brigade.
It's all really so there's some form of plot to add continuity to the last game and give you a
shape the world type of power fantasy. Even if they are the most supreme assholes in the universe, even an asshole doesn't much enjoy it when you come into their home and set it on fire. It doesn't matter if the one holding the match was an ant, that just means they're going to get stomped by the bewildered home owner instead, especially when it's threatening to shoot your dog next.
TLDR-there's not much actual justification that works for the gods not beating up Kassyra immediately, it's just a hastily put together plot device.