The Laundry Files series describes a setting where enough computation and the wrong algorithms produce magic, where humanity is inevitably doomed by the impending Lovecraftian apocalypse from Moore's Law and population growth. Secret occult intelligence agencies try and forestall this as best they can, with their signature equipment including smartphones with apps installed for warding, exorcism, disguise, and geases.
One fic idea that came to mind from reading the series was of somebody else, like a date rapist or phone repair tech who routinely clones phones for blackmail material, getting their hands on one of those devices and kicking off a hypno app story.
The first part would cover them getting the device and their initial experiments with it, and an amoral pursuit of sex and money and revenge for old petty scores. Then would come the occult consequences, maybe a long-held thrall getting possessed by something else and almost killing the protagonist, forcing them to track down the person they got the phone from and work around their wards to hypnotize them and find out what's really going on. Not sure how to end it after they learn about the impending apocalypse, however. Perhaps they relaunch into their rampant amoral hedonism, knowing the world will end soon, with a new hollowness to it? Or turn to vigilantism, trying to cover the gnawing dread with the thrill of self-righteousness? Or collect magically important people, starting with the one they got the phone from, telling themselves that their harem is necessary for defending themselves and others?
In terms of theming, the main ones that come to mind are agency and coercion, and how a large enough looming threat can make both other consequences and the threat itself seem unreal. Maybe the lead has a past of crimes that hang over them, that they're already half-expecting to be caught and sentenced for, and they engage in cycles of escalation only to be brought down by something mundane? Class isn't a theme I've explored very much either, and this feels like it could be a vehicle for that too.