BIG WARNING!!!! DEATH IS MENTIONED AND CONSTANTLY TALKED ABOUT IN THIS POST!!!!
Okay, with that out of the way.
Idk how to put this into words the way I want to, so I'm just going with: this is how I headcanon the death of a deity in the phighting universe would be like.
Everything under the cut so those who want to scroll past this don't see a long af post about the deaths of the deities and stuff like that. I really rambled bro.
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Okay so quick explanation before I get into the actual headcanons part: Killing a full deity (so this cannot happen to demi-deities because they can be killed normally, they just cannot die like other normal mortals can) is a very rare occurance and does not often happen. It only happens when a mortal bonds so closely with a deity (some sort of close/intimate relationship, doesn't have to be romantic just in general) that the deity attaches themself to the mortal physically. This attachment will start to slowly kill the mortal, which in turn will start to kill the deity as well. It isn't intentional and unfortunately cannot be stopped without the complete separation of deity and mortal (or if the mortal is killed before the whole process completes itself, whether that be from another mortal, the deity, sickness, or other natural causes).
I headcanon this is how the non-SFOTH deities came to an end (for now, since it hasn't been revealed how they really did).
And now for how I believe it would happen to the SFOTH (ordered in a way I remember them all, this order is actually very important to me for reasons).
Venomshank
The mortal is slowly poisoned. Green venom spreads through their veins. It kills the mortal once it reaches BOTH their heart and brain (what is Inphernal biology do they have internal organs like humans, I will write them as such until stated otherwise). It's not a quick process, but takes place over the course of 2 months.
Venomshank himself is affected as his muscles start to paralyze wherever the venom affects his bonded mortal.
I think for Venomshank, it would be kind of hard for him to separate from the mortal. He cares a lot when he gets close to someone, so yeah it would kinda suck for him to do it. But I think he could if he convinced himself that it's better for the mortal.
Extra headcanon: could happen between him and Sword. He cares so much for his mentee, he would do anything to keep him safe. So much familial love for this Inphernal he picked up as a newspawn, an Inphernal he views sort of like a son at this point. He wouldn't want to separate himself from Sword, but if he found it necessary then he might hand Sword off to someone he trusts (Zuka is the one that comes to mind, but could actually be just anyone Venomshank trusts in general to watch over Sword in his place).
Darkheart
The mortal is slowly consumed by shadows. Black tendrils of darkness start at their extremities (fingers and toes) and start to spread upwards and outwards. It kills the mortal once it fully consumes them. It takes a little over a month for the process to complete itself.
Darkheart is affected by ONLY being seen by said mortal. Nobody else can see, hear, or interact physically with him. This makes it harder for him to leave his bonded mortal because (to me) he's quite the social being. His body is never found after the mortal dies.
It might not be hard initially for Darkheart to separate himself from the mortal, but if he lets it progress too far he won't want to leave the mortal behind.
Icedagger
The mortal starts to freeze and frost over. It would really depend on where it starts, but would most likely start at the hands. It kills the mortal when they get cold enough to succumb to hypothermia. This may take a while, it's VERY slow. Maybe a year, give or take a few weeks. Very slow process.
Icedagger actually ALSO starts to freeze over. He doesn't notice it on himself, but definitely on the mortal. He doesn't know this can happen, so he thinks it's fine until it's too late. When the time comes, he freezes both himself and the mortal in a block of ice to preserve both bodies. If someone he cares about is dying, he'd rather they be untouched by the elements.
Most likely to die with the mortal. He's lonely, wherever he is. He longs for connection. Once he has it, he isn't giving it away. You know the wedding vow "Till death do us part"?? Yeah, that's Icedagger except with any connection he gets ever. Any type of relationship he forms, he won't give up without a fight. If he's dying with a mortal he holds dear, then he'd die with them willingly.
Firebrand
The mortal heats up from the inside until they get so hot inside they just start melting. It starts at their heart, spreading outward but growing warmer there until it kills them. This process is probably the shortest of all of them, taking just about two weeks to completely kill the mortal.
Firebrand starts to lose the heat in his hands. That's how he knows. This has happened before, but he found it easier to leave for his own safety. This was before his child, but once he spawned his kid he was super scared of it happening with them, and was super scared of it happening to his grandsons too. He loves his family dearly and he doesn't know it can't happen to demi-deities (which was why he was scared in the first place).
He would be able to find it in himself to leave. It would be hard, yes, of course it's hard to leave someone you care about. But he knows it's for the best and so he would be able to pull away. He's done it before.
Extra Headcanon: before The Scorch, it was starting to happen to Umbrella. They had made the small complaint of feeling a painful heat in their chest around the same time Firebrand had accidentally picked up something without his gloves and noticed it hadn't started melting when he did realize. He started freaking out. This has nothing to do with the actual cause of The Scorch, but The Scorch separating the both of them was better for both of them to stay alive. Firebrand has told Windforce in one of her visits to wherever he is now.
Windforce
The mortal becomes pretty much a lightning rod. If it's storming outside and they aren't inside, they have so much of a higher chance of being struck than anything else around them. This isn't much of a process more than it is noticing how much more conductivity (is that the word I'm looking for) the mortal is gaining before it's too late. Being struck by lightning once (depending on where the mortal is struck) won't kill them. But it'll hurt and suck a lot. But multiple times could probably do it, so going outside when it's storming is NOT a good idea. Also the mortal gets shocked (those shocks when you touch someone or something and you get the little zap) a lot easier and has to be a HELL of a lot more careful around electrical stuff.
Windforce is unaffected. Probably the only one to be fully unaffected by this until the mortal dies (she dies with them, obviously).
She'd have an easier time leaving. She has a son to care for, whatever relationship she has with the mortal is less important than the duty of parenthood. She's also the leader????? Of the Splintered Skies faction. She can't let them go without one. She also might just end up killing the mortal, though because she does care about them she'd make it quick and/or painless. I don't think she'd draw out the death of someone she cares about. Windforce is fucked up but not that fucked up guys c'mon.
Extra Headcanon: this most likely wouldn't happen with her, given how she doesn't seem to like mortals (from what I've gathered).
Ghostwalker
Preface Headcanon: Ghostwalker, in canon, doesn't feel regular emotions. I think he feels emotions in a different way than perceived by the normal person. So his "care" is different than what a mortal would feel for "care". He may attempt to replicate a mortal's version, but he feels it differently than they do.
Similar to Subspace's rot, the cells of the mortal just start to die and stop reproducing in areas. The starting point of it varies. This process can take SEVERAL years, and while it can be stopped, can never be fixed. Cells won't reproduce in some areas, leaving the mortal in a similar situation to Subspace (missing chunks of flesh, bone open to the air, aching in areas around the cell deadzones).
Ghostwalker becomes weaker in the same areas rotting on the mortal. Even if the process is stopped, he'll still have that weakness until the mortal dies naturally (if he were to leave them).
He wouldn't LEAVE the mortal, but would probably kill them. A quick, as painless as he can make it death before he moves them to the afterlife with other mortal souls.
Extra Headcanon: this cannot happen between Ghostwalker and any of the Watchers. He created them, and while they aren't demi-deities or deities at all, they aren't exactly mortal Inphernals, either. He made them specifically in a way where it would not happen, and allows himself to care about them (the way he cares).
Extra Headcanon: this has happened before to Ghostwalker as well, but he said nothing of it and killed the mortal himself. It was before the Watchers were created, and he decided when he created them to make them so this wouldn't happen with them.
Illumina
Well, first the mortal gets sparkly. Kind of like if there were glitter coating their skin. Then it gets brighter and brighter as time goes on, only stopping when the mortal starts to actually glow. Then, a second stage starts. Their skin starts to harden into stone, and once fully encapsulated, they'll look like a marble statue from the outside. The process can take about 6 months.
Illumina himself isn't affected until the second stage. He also starts to turn into stone. If the process is stopped, he'll have to chip off pieces of stone left behind. It hurts, but will heal eventually.
He won't want to leave (when he forms a connection, it's a very very strong one), but forces down that feeling in order to keep himself alive. While he doesn't like some of them (especially Darkheart), he has to be around for his siblings.
Extra Headcanon: he's aware this can happen, has seen it happen to a non-sfoth deity before, but tells no one of his knowledge. He's scared of it happening to him, so he keeps EVERYONE at an arms length, even any of his siblings he's close with (currently Ghostwalker, since the both of them are working together or something idfk I know in canon they are I just don't remember why). He knows it can't happen between two deities, but just in case. And he doesn't like to interact with mortals because of this, but will interact with his followers with a sort detachment (not viewing them as people helps him I guess) so it's easier for it to not happen to him. Poor bro is terrified someone please hug him.