...What. Even as a ploy, that's completely ridiculous. Not the infighting between demons; really, it would be in some ways more surprising if Areelu wasn't stabbing her master in the back when she thought she could get away with it. But if that was the plan, it wouldn't involve admitting her involvement in a way that Deskari might be able to later learn about and would make more sense. In the incredibly unlikely event that a demon lord would decide to manifest on the material plane at a predictable time and you wanted to betray them about it, you would tell Lastwall so they could have a few dozen invisible paladins waiting in the wings to kill him before he has a chance to respond, not give Terendelev a bit of warning but not enough to actually matter. But that still leaves unanswered the question of what the goal of the intervention is. For Terendelev to blow her spells preparing for an attack, and then kill some patrols when she can't support them because she's out? Possible, but probably not worth the cost of revealing you have a high level wizard under your employ and risking losing them. To get her to cancel the festivities out of paranoia? Even less payout, especially as demons tend to reckon things. Get her paranoid and have her bug out when you trigger a normal demon attack, leaving the city without its best defender? Possible, and it might even work, but the story is still pretty confusing even if you assume that. Terendelev is beginning to feel lost, like her conversational partner is playing at something she's only half understanding, and it's pretty uncomfortable position to be in.
"I do have some spells that I could use to prepare for an attack, but most of them have a fairly limited duration, so if I cast them too early and often I would run out before too long. Did you happen to get a time? And, ah, I'm sorry to doubt you, but this story sounds rather unbelievable and that's giving me some issues, and you seem more than capable of faking most of the things I could check. Is it an immediate emergency, or do we have time to bring you to Rathimus and see about having him cast Abadar's truthtelling on you? That wouldn't erase the doubts, but it would help rule out some of the possibilities."