Carissa's plan to solve that problem is going to simply be 'don't be tortured into giving consent for a resurrection so the Queen can statue you'. It seems like an incredibly solid plan. 

She prepared defensive spells, not offensive ones, or this would be as simple as putting her fist in her face with a Fireball; as it is she's in fact going to need a knife. She goes invisible.

This palace does still contain a certain number of people who are pretty fucking likely to spot an invisible person walking around, unless that person is very invisible.  On the other hand, if they're very-invisible palace Security themselves, they won't necessarily stop you right away, after they've spotted you.

Well anyways, this could be funny, how is Carissa going to try to kill herself?

She's not really thinking all that clearly. But she has her Bag of Holding with the supplies with which to protect Keltham. So she can be Greater Invisible, and have a Nondetection up, and that ought to keep her from being immediately stopped while she thinks. Options: acquire a fast-acting poison, acquire a knife, attack someone in such a fashion that you get killed by responding Security, hope she can hide long enough to make an arrow of Slaying - no, that'd take a full day, she doesn't have that long.... oh, no, wait, all that is stupid overthinking.

She finds a fountain. 

There are lots of them, dotting the grounds; she doesn't think she needs the basin to be more than a few inches deep, in principle you can drown in practically anything, but she picks one with a few feet deep, just in case her self-control while drowning is much worse than she thinks. She blankets the fountain with an illusion that it looks normal. Lies down. 

Lowers her head under the water, and breathes in.

Most people, in fact, would not be able to deliberately drown themselves like that.

People suck. It is why Asmodeus hates them, and Asmodeus is right to; Carissa agrees with him wholeheartedly. Theologically everything feels very uncomplicated, right now. People are weak and pathetic and can't think about the long term, can only think an instant ahead, can only feel their lungs fill up with water and jerk above the surface, their muscles moving in advance of their mind.

If she's that weak she will deserve to stop existing. 

She lost the Minor Illusion. She casts it again.

She sinks down into the water and wedges her head slightly under the lip of the fountain as protection against the failure of her own will, and she focuses all of her self to a tiny sharp point on the Minor Illusion. She has to keep it up. The fountain has to look normal. Her lungs are spasming and her body is fighting but this is the only thing that will ever matter and she doesn't move above the water, doesn't keep track of which direction is above the water, and the illusion will make the water look normal. The illusion will make the water look normal. Perfectly normal. Even when the illusion falls, an invisible body won't be fast to find, but she is trimming away even thoughts like that, now, in favor of concentrating on her illusion's sparkling beautiful perfection, nothing here for anyone to see. 

It's getting dark, but that's all right. She'll wake up in Hell.

Technically, she'll get a trial, which is almost funny, in some very distant far-off way. 

Pulled out of water by a Security not wearing an intelligence headband, tapped with Greater Dispel Magic (cast from item) followed by Greater Infernal Healing.  "What happened to you, report!"

Wait what the actual fuck, she thinks while coughing and vomiting on the ground. We have security competent to notice invisible bodies in the fountains but not to, you know, detect a Sending or wake up the one on-site project expert when an important target is acting weirdly or brief anyone on the escape plan -

"I have an important report I need to make directly to Dispater," she says. "In person. If you haven't got training for this I will take your knife and handle it."

"That's the weirdest thing I've ever heard in my life, and if it can wait long enough for you to make it to Dis, then it can wait slightly longer while I get in somebody who can check you for enchantments."  He's pretty obviously calling reinforcements.

"I'll go directly to Dis, you worm, I sold my soul there."

"Aura Sight doesn't have you looking powerful enough to have done that.  Fail your Will save."  Knock knock, somebody would like to detect her thoughts.

Instead of that she casts Hold Person. And then Invisibility, to hide the frozen Security somehow standing right there, and then - has he got a weapon on him, or items more lethal than the stupid defensive ones she has -

Mm, good question...

There's a dagger that looks kind of unfortunately ceremonial, barbed, and pretty blunt aside from being barbed.  You might be able to kill yourself with this but it would be painful and, more importantly, slow.

Fine.

She shoves him into the fountain. Unpleasant way to die but Hold Person doesn't last very long and this way she can determine whether there's some kind of drowning-person alarm on the fountains, he got to her way too fast. 

(She casts another Hold Person from the scrolls in her bag when it reaches the edge of its duration.)

Alter Self. Disguise Self. Now Carissa is this random Security.

Brilliant deduction, Carissa, it looks like there's more Security inbound!  Either that, or the first guy called for reinforcements just like you remembered him doing!  By the way they probably know him, also how well do you think you're likely to do at faking his Arcane Mark.

Carissa, in the manner of dath ilani, updated hard on the quality of the remaining Security in the last hour and is now pretty confident that everyone who would be smart enough to check that is either in Nidal or, uh, deserted. It's occurring to her that possibly a lot of the absence of competent people is desertion. - think about that later. 

"Two invisible people, one of them a caster, grabbed one but they Dim Doored out," she shouts to incoming Security. 

...okay, they'll start casting detection spells that will totally show up that Disguise Self you're using.

She has a second Nondetection which ought to cover that! She'll cast some detection spells herself and go looking for the intruders. There should be some non-caster Security she can steal a knife from.

One supposes that could work.  As interesting as it might be to watch Carissa try to end herself with a knife, however, it looks like the alarm has gone up and also some more competent-looking people are returning from the front lines to deal with whatever's going on at the Palace; Chelish forces can in fact handle themselves pretty well against Nidal if the Black Triune is out of the picture.

Also somebody may possibly have reported that whole thing with Keltham at some point?

Somebody is yelling about whether anybody knows where Carissa Sevar is and she's to be held alive if so.

Unfortunately you didn't do it exactly right, then; or even, really, right enough at all.  Not enough to prevent yourself from being healed after they find you.

There's an instant of blinding pain as she tries it.

It is alas followed by waking up, not in Hell.

Carissa is, as sincerely as a person can possibly be, really sad to not be on fire in Avernus.

Where is she?

The Imperial palace's best torture chambers; apparently it's time for an extended debriefing which will probably be, in fact, pretty boring from the standpoint of the people doing it -

Ha ha, just kidding!  Carissa is nowhere near that lucky.

She finds herself in a place that involves a remarkable amount of golden shiny, which is the color of the walls, floor, ceiling, all illuminated by ceiling lights bright enough to look like little pieces of Sun.

At various forcescreen-protectable desk stations around the room are unidentifiably rare books, alchemical reagents at the intermediate level, large gemstones in more colors than the rainbow actually has, exotic metals, and one shelf with an absolutely horrifying amount of spellsilver.

There's nobody visible in this room, but Carissa can't turn her head very far to see if anybody's lurking behind her.  Her feet are spread uncomfortably far apart and affixed to the floor by anklets and chains that don't have very much give to them, her head is affixed in place by something that feels more like an arbitrary force spell.

Gold, to be clear on this point, is a usefully non-reactive coating; and hence often found all over the place, in very very high-end enchanter's workshops.

How is Carissa currently feeling about this?

Carissa has gone so far beyond all of the emotions she has ever had in her entire life that when she checks where the emotions are, she just comes up empty. 

.....it is technically possible to use spellsilver you're not touching. Some people do that so they don't have to deal with it oxidizing. Doing it at any significant distance loses efficiency, which wouldn't be worth it, but -

She is going to focus very intently on trying to build a weapon-enchantment-scaffold that stretches all the way from her to the spellsilver on the shelf. 

...isn't she supposed to be third circle.  Is she still third circle, she did just get into a bit of a fight there.

Well, nobody's going to interrupt her while she's trying that.