Tony sits in the dark on his sofa, looking down onto the city lights of New York and tries really, very hard no to start crying.
Not because he's sad, or mourning.
It is the frustration that seems to have dug itself into every stupid, mortal, dying cell of his body.
He is frustrated that because even if the math works for him, the math and the science, and the tech and even his fucking basic genetics are fucking perfect for what he's attempting. Really, it's all right there, and it all sais that it should work, that he should be able to live at least another five hundred years, but whatever he does, it fails.
The closest he got still was a serum and extremis combination, a modified one from what Kilian had thrown around obviously, but still extremis. Except that the tests showed that for all it's healing-factor like abilities, they can preserve his body, but they can't preserve his brain, and the longer he'd live, the higher the probability of illnesses as Alzheimer and the like.
Well, 'probability' is the wrong word.
Tony will not lie to himself and deny that at fifty-nine, his brain already isn't what it was with thirty. He was always prone to forgetting things he wasn't interested in, or didn't work or had to do with on a regular basis (Hammer's face, birthdays, and let's not get started on Pepper's strawberry allergy).
Well no, he used to ignore those things, not forget them.
Yesterday he forgot that he'd started fixing the coffee machine of the workshop (and still couldn't remember that he'd done so when Jarvis reminded him).
It's terrifying to have his strongest point slowly fail him bit by bit, and be aware of it. Knowing that he can't do anything against it, knowing that it will only get worse, much worse.
Perhaps it was better that Loki had left before the slow deconstruction of 'Tony Stark' really started.
Before 'they' really started, though it was stupid to deny that there had been a 'they'.
They had never talked about cloying and binding things like 'feelings' and subsequently never labelled themselves as anything beyond acquaintances, but Loki had lived in the tower for almost ten years (one of them as exile for letting Jotun into Asgard, the rest voluntarily), and during that time they had first bonded over technology, magic and mathematics.
Magic had remarkably much mathematics in it.
So what started as 'annoying each other because of boredom' via tablets between Loki's room and Tony's workshop, led to geeking out in said workshop and blowing things up.
Even if Tony wouldn't have minded jumping the God of Sex (that mind alone, Tesla save his soul), Loki was the not quite God of Sex, because Loki doesn't do sex, and he could totally accept that.
Tony found out the when they'd both fallen asleep in a drunken stupor on the workshop's couch, and Loki's reaction upon waking up shirtless on top of Tony had been less than stellar, and resulted in the god (at that time human) having a complete freak out and locking himself into his room.
It had taken Tony a whole week to put together the pieces and realise that sex was an issue for Loki, and another two to catch the god alone and get him to talk about this, because there is really nothing wrong with not wanting sex or sexual contact of any sort, for whatever reasons one might or might not have.
Turns out that pretty much all Realms see that as some sort of defect and all have their own sort of 'curing' it, and that there'd been a number of gruesome 'almost' events.
But once Loki was convinced that Tony meant what he said, and had been introduced to Rhodey (the other asexual in Tony's life), the god was an extremely tactile creature.
Loki became something of an overgrown, too smart, godly house cat (Tony had taken to calling him 'Bastet', the Egyptian cat goddess), and his favourite past times became mussing up Tony's hair and using him as a footstool or seat, and even as 'blanket heater' once he decided that
Tony was trustworthy Tony's bed was more comfortable.
One of those nights the god (human) had asked him "Does it ever get easier? Constantly dying, knowing you only have a few years left?"
It took Tony quite a while to make himself answer truthfully with "No.", because it didn't.
Not for him, not for Loki.
The first knowing he'll never live long enough to achieve everything he could or wanted to, the other never before being confronted with the concept of mortality beyond observation.
Four weeks later Loki regained his godhood to save the team from an avalanche with is magic.
Not much had changed afterwards, the team was still making weird jokes about them that both, Loki and Tony, ignored in their own ways.
Let them think they had sex, it's not like it's any of their business, or as if they were in a relationship.
They just enjoyed each other’s company and intelligence.
The only things that did change was Tony's magic based research, the frequent minor pranks played on the other Avengers (minus Natasha), and that the god frequently left Earth, returning after days or weeks, usually in subpar shape, but at least he returned and even called the tower 'home'. Which was something Tony completely refused to pay closer attention to.
Tony's projects for immortality (or at least any form of extending his life) had started long before Loki had become a god again, and even if he'd never even shared the idea with him at any point, Loki knew they existed, he was quite sure of it.
But Tony quickly realised that anything beyond thirty years extension would take complete physical remapping, and that complete physical remapping was almost impossible without damage on someone older than three.
Tony'd been fifty-four when Loki vanished into thin air, and fifty-five when he returned.
At first Loki seemed relieved to be back, or so it seemed on the security footage, until he caught sight of Tony, whose temples and beard had rapidly turned grey within that year’s time. Age was catching up with him rapidly and Tony wasn't surprised one bit, he'd kept well until that point.
He'd greeted the god teasingly, just as he always did, but instead of throwing a jab back Loki had looked at him with shock, realisation and then a bitter (unconvincing) mask.
"You look old." Loki sneered, and all Tony could do was laugh at him.
"Don't act so surprised, you know I'm dying just as much as you were, if not more."
Tony had known this would happen.
He'd known, because for all his efforts he is just a mortal.
It still hurt.
Because for all that they weren't, had never been partners (That's a lie. Tony's mind provided), they'd at least been friends, or so he'd like to think, and therefore Loki's hissed "You are no use to me dying." was as effective as any good old dagger stabbing him in the back.
The god was gone, long before Tony's bitter (broken) laugh stopped. (Because if he didn't laugh he would've ended up crying because it's true.)
Tony shook his head to get rid of the memories from over four years ago.
There was no point in dwelling on Loki, on those years and years with, what Pepper jokingly called, his 'soulmate'. As close to a healthy relationship as Tony had ever gotten.
It was ridiculous that it'd been him who'd fucked up again.
"God it's so useless." he sighed as he dropped his head onto the sofa lean, letting his eyes fall shut.
"Sir?" Jarvis' voice asked, sounding almost unsure, but Tony couldn't resent his AI for it. His mind's ways were getting erratic, so that even someone as acquainted with his off tangent thoughts as Jarvis could barely follow him anymore on bad days.
"Living longer, Jarvis. Whatever I do it doesn't work even if it should. Doc said I've got another decade in me, probably not a second."
There was an untypically long silence, almost lasting for a couple of minutes until Jarvis spoke back up, this time sounding really unsure. "Sir, what am I to do when you are..."
"Dead?" Tony finished for him, "Whatever you want. Tell me when you've got a plan together so I can help you get it done, there's no hurry just ye-”
The rest of his answer got drowned out by a resounding crash from the kitchen and subdued swearing in a voice that Tony hadn't actually expected to hear (ever) again.
It took Tony longer than he wanted until he was standing in the kitchen door (fucking god damned fucking knee) and found Loki lying in the middle of the floor, breathing slowly and heavily, and bleeding all over his floor, smearing it on the light wooden panels of Tony's counters where he was pulling himself up.
"Fucking hell, what happened to you?!" Tony swore, protesting knee forgotten and the 'I should be mad at him.' ignored in favour of getting the god to sit down on one of the kitchen chairs, and find some medical appliances when he saw that Loki just let himself slump back and be absent.
Awake while unconscious. Something Tony’s seen on him before when severely magically depleted, but never in combination with this much blood (or cuts in his armour) since Loki usually healed himself with his magic.
Thank god, in this case literally, Loki woke up the next morning, hands still wrapped around the miniaturised Arc Reactor to replenish his magic (something they'd found out not long after said magic had been regained) and looking a bit worse for wear, but Tony expected that the worst wounds had already healed.
Before Tony could even say anything upon entering the room (trying his best to hide the limp, his knee had not been impressed with dragging a god with hyper-dense tissue through the penthouse), Loki was on him instantly, arms tightly wrapped around Tony's shoulders and face buried in the man's neck, and for all that Tony wanted to shove him away, for all that Tony wanted to yell at Loki 'I'm still dying' and 'How dare you leave just like that!' ( or 'Please god, never leave again, I missed you!' ), all he did in the end was hug back and relish the few minutes he'd have before Loki would come to his senses.
Loki slowly stepped back, but didn't let go, and the way he looked at him, green eyes full with regret, guilt and something else that might pass for triumph, Tony was pretty sure that the god knew exactly what mixed reactions he was having.
"I have something for you" Loki whispered as if it was a secret, and reached into one of his spaces 'in between', pulling out something that looked like a cross of a fig and a plum, except that it was a much too bright purple to be either (or anything else Tony'd seen fruit or vegetable wise on earth), "I know you strived for immortality for a while now, and I ... I can see that you failed. This," he put the fruit into Tony's hand where it turned out to be surprisingly heavy,” can change you into an immortal, if you wish it so."
Tony just stared just between the- oh whatever let's just dub it a 'fig' and Loki for a moment, realising what the god just handed him.
His ticket to life.