Contrary to what everyone had assumed at the time, Xadhoom had been both alive and fully conscious, after she'd left that ship, and had ignited her body, into a new sun, for the surviving Xerbians.
She would've had to, in order to continually provide energy and fuel for the star. She assumed that Xari had worked it out, and presumably, the Avenger and One had connected the dots as well at some point, though she had no way to be certain of either assumption.
A few quiet years quickly passed since then, with no visitors, hostile or otherwise, until a spaceship approached Xerba's system.
To her surprise, they weren't there for Xerba, but for her, as they apparently had some kind of technology that allowed them to drain energy from stars...
As they would quickly find out, she did not take kindly to being robbed.
About half a decade after that odd incident, as she watched the system, scanning for threats to the Xerbians, from within the star they'd named 'Xadhoom' in her honor, which she was greatly flattered about, she saw the surviving colony leave their planet, on a worldship, making it safe to extinguish the star, re-absorbing the energies she'd released in generating it back into herself, once they were sufficiently far away.
Hopefully, without any hostile races like the Evronians to cause trouble for them, and anyone to be as idiotic and stubborn as she'd been, they could finally live happily ever after...
After that, she spent a year in space, before returning to Earth, to the Avenger, and One, and this 'Lyla' person, and occasionally giving him a helping hand, fighting assorted criminals, and occasionally flirting with him, as they had during the Evronian war, which eventually grew into a full-fledged relationship.
It'd been a good decade or so, until they eventually had to confront the fact that he was aging, and she wasn't.
Reluctanctly, they ended the relationship there, and she left Earth once more, returning infrequently, eventually staying a while, for the funeral...
"That was over two-hundred years ago now, as measured on Earth..." Xadhoom sadly mused to herself, lying on an asteroid, in some distant, nameless constellation, bathed in the soft light of what humans had termed a B-class star.
Despite the passage of centuries, she looked no different than she had, when she'd fought the Evronians, countless years, and driven them into functional extinction. It seemed that she had cursed herself with more than an unstable set of superpowers...
She understood the physics behind her immortality full well; energy-beings, even ones that assumed biological form, weren't exactly susceptible to the rigors of aging, she groaned, running a hand through her soft hair. Why so many among the biological races sought it, she had no absolutely no idea, as while it did have its occasional advantages, it came with a considerable number of drawbacks, chief among which, a considerable lack of entertainment.
She just wished she could follow the Avenger's example; in his short lifespan, of seventy-three years, he'd accomplished infinitely more, than she had, in centuries... "Really, mortals don't know what opportunities they have." That she'd referred to them as 'mortals', completely escaped her...
Deciding to continue her musings on her past, she fondly remembered spending a decade or two, roaming the galaxy, visiting various alien cultures, of which some had been hostile, some had been peaceful, and some had, like Earth, been strange, terrible, and wonderful.
However, that had only proved a temporary diversion, which forced her to once more seek out alternate means of relieving her boredom.
Re-joining the Xerbian community, aboard their worldship, had proven fascinating, though the impossibility of any sort of romantic attachment had rendered the experience inherently limited, and she'd only been able to spend a few short years there, before her non-existent aging raised too many questions.
With nothing else to do, she left Earth's galaxy behind, to explore the wider universe, and fight assorted evils, some of which had actually managed to challenge her somewhat. It had been an amusing way to spend a century or so, but eventually, even that had become tiresome.
She'd briefly entertained the notion of traveling to other universes, to see whether she could get a decent fight, and new experiences there, but ultimately decided against it, as she might never be able to return to her own universe, if she did.
After that, she deemed it a good idea to find an immobile asteroid somewhere, and speculate about her place in the universe.
After about a decade or two of introspection on there, she paid a visit to Earth, to meet up with 'Odin Eidolon', as she learned One now called himself, and remiscinisce about old times, with the Avenger.
Being a fellow immortal, he understood her boredom better than anyone else, and had offered the Xerbian a position in the research department of his business venture, to relieve her of it, but eventually they agreed that tying her down to a single planet wouldn't solve that problem, and she departed again.
Though, she continued to visit him a few times every decade, over the centuries, hellbent not to let this tenuous tie to a planet, and an individual, snap, or fade away, as neither Xari or the Avenger, whether as her friend or her lover, would've wished that for her.
This tentative association continued into the 23th century, where she decided to re-locate to Earth once more, as with Xerba abandoned, the Xerbians alive and comfortable on their worldship, and the Evronians long since gone, she had no real reason not to stay, though she knew, as Odin did, that she would have to leave the planet, whenever the Avenger visited, so as to not disrupt the space-time continuum.
Privately, she doubted the not-quite-as-composed-as-he-let-on machine sentient's ability to keep past and present separate...
...
Briefly, she wondered whether she had in fact gone mad; for one to spend centuries in isolation, drifting aimlessly through space, with no companionship, other than a machine sentient that they only met every few years, certainly couldn't exactly be the healthiest things for one's psyche, she laughed.
Suddenly, a series of soft beeps interrupted her musings, coming from what she recognized as that portable holocom thing Odin gave her a few visits ago, signaling that the Avenger had returned to the 21st century, and that she could safely return to Earth.
Okay, so the timeline of this is: # 37, the later events of the series, PK2, post-series.