The alethiometer talks to him for a while about this one.
"There's a spell for it," he says, watching the needle. "Or there... could be? It's never been done but it's possible. It'll take... everything. Everyone. Sacrifice, herbs, runes, verse, every goddess, and... you said that part already," he tells the alethiometer in fond exasperation, "nobody's ever done this before, I get it—oh. And something nobody's ever done before. A new kind of spell. A way of doing magic that's new. On top of all that other shit."
He looks up at Isabella and smiles crookedly.
"Have fun with that."
"Okay. And otherwise lots of painstaking research. I'd probably better start with something smaller than immortality. Don't you die anytime in the next - well, ever, but especially not before I figure this out at least as well as I know how to do verse and herbs and runes and sacrifice."
"Well. That could require some slightly heavier duty protections than if what you had in mind was the possibility of being hit by a cement truck while jaywalking," muses Isabella. "I think the gold standard for witches who want to protect mortals who are, say, soldiers, involves a tattoo. Of a bayleaf, with bayleaf-infused ink, ideally one on you and one on Petaal, with a verse said over it. And when I say a tattoo I don't mean one with needles, I mean the relevant verse makes the ink sink in all by itself after it's painted on. It does still hurt about like getting a regular tattoo, though. And that'll let you last through serious injuries longer before you get medical attention, although not forever, and I can't think of anything that would help if someone's priority were to make you dead and they actually checked. The tattoo would be obviously a witch tattoo to anyone who saw it and was the least bit magically sensitive, though, they'd know they'd piss me off if you died and it was their fault."
The ink contains many dubious ingredients - the cornucopia doesn't care about processing time and is able to save her awhile by producing powdered bayleaf and the other requisite herbs, and the rest of the ink is vinegar, ash retrieved from the firepit, and one of Isabella's tears (she produces this with the expedient of Path biting her ear).
She fetches a paintbrush, and murmurs her animal-summon to get ahold of a seagull, which she holds in her left hand, ready to snap its neck, while brandishing the brush with her right. "Where do you want it?" she asks.
"Nice," says Isabella, peering at him speculatively. "You'll have to lie down, then. Do you want yours to match, Petaal? It'll be easiest to paint yours if you're in this or human shape, although it'll persist however you change, it's adapted to work on children just fine."
"Bayleaf, bayleaf. Safe and sound,
Keep my precious pair with me,
Live and whole and lucid, leaf,
That is how I need them both,
Bayleaf, bayleaf, guard them well.
Bayleaf, bayleaf, hear my verse
I protect and mind with you,
Hide inside them, safe and sound.
Burning down into their skin.
Bayleaf, bayleaf, guard them well.
Bayleaf, bayleaf, I am one
I remain at home, they go
I am safe and they in peril
Danger you will chase away
Bayleaf, bayleaf, close their wounds
Bayleaf, bayleaf, help them breathe
Bayleaf, bayleaf, they are mine
Bayleaf, bayleaf, do as asked
Bayleaf, bayleaf, guard them well,
Bayleaf, bayleaf, safe and sound."
On the last syllable, the seagull dies in her hand, and the ink sears itself into matching tattoos.
"Ooookay," laughs Isabella. "That'll keep Metis off my back for a while if she starts pestering me about curses even after I've killed a guy." Pause. "By the way, killer-dance-moves magic - I think should generally be a secret. I'm already occasionally theatrical when I do other spells, I'll start throwing in random movements where they aren't needed and then if she sees me doing it to different effect she won't know it's special."