The Heartless wouldn't eat them (and Demyx's prissy Nobodies wouldn't touch 'em).
However, the same did not apply in the inverse.
The zombies thought the Heartless were delicious and the Nobodies tasted like chicken.
They said so, in that croaking jaw-rotting-of-hinges undead sort of way.
The first thing Roxas did upon finding this situation was to go kick in Xemnas' study door. Accompanied by all the other neophytes he had rallied.
"Hey. Xemnas. If we don't exist? How can we be wiped out by a zombie-horde! You stupid, asshat!"
Xemnas was impeccably verbose when he said, "Shit, I don't know."
Due to a popular vote of no confidence, Xemnas was sent out to talk with the zombie general. Predictably (entertainingly) he was returned somewhat gnawed upon and with much less flesh.
However, oddly enough, his Nobody brethren were neither bereaved, nor concerned. In fact, they were curious as to why the zombies sent him back at all, since they then proceeded to toss their once-upon-a-Superior back out of the Castle from one of the higher Ledges of Neither Here Nor (Paradoxically Dramatic) There.
They left his broken body upon the front walk as a warning to anyone else who might want to get uppity.
Roxas stared blatantly at… at well, everyone.
"If any of you try anything before these zombies are taken out, I'm going to kill you."
They all looked down at their feet (except Xigbar) shamed to have been so transparent.
"Always had to be the nonconformist, didn't you Xigbar," Xaldin hissed with disdain.
"Hey, hey, Roxas!" Axel called.
Roxas rolled his eyes and turned on heel. He could hear the rest of the Organization waging zombie warfare, with limited success. Their troops kept being eaten and a select few of them had begun to come back, their once silver lining and existential white bodies turned putrid shades of green.
"What is it." Roxas acquiesced with resignation. Escaping from Axel when he felt the need to be an obnoxiously persistent prick was nigh on impossible.
"Well," Axel slunk in close, touching far too much for Roxas' taste, so the boy slipped out of his grasp again. However, Axel was in the mood to be an obnoxiously persistent prick and tried again. "I was thinking you and I could escape, just the two of us."
Roxas stared at him dryly, waiting to see if the stare would have any effect. When it didn't, he rolled his eyes again.
"And a-band-on our dear sweet com-rades to their de-aths, Axel? How un-think-able." Axel dodged the first punch, which made Roxas ver angry, so the next six landed with painful perfection. "What part of 'if any of you try anything before these zombies are taken out, I'm going to kill you' did you misunderstand, moron? Since you haven't actually done anything yet you can get off with that, but… God, you're stupid!"
(He didn't want to go off into the sunset with Axel anyway. He was a creepy pervert. Jesus, Roxas had only been in possession of his own consciousness for a few months. Cradle Robber.)
Grave robbing suddenly became a viable weapon for the zombie siege. Putting Xemnas down again was a pain in the ass, because even as they hacked him to bits, he was very insistent about fighting on. And gnawing.
He got Marluxia.
Left a big festering wound on his face, which Vexen examined and compared with the samples they'd brought back from their turned Dusks.
"The process is slower in our larger bodies, but his transformation is inevitable," Vexen had said.
Maybe it was true and maybe it wasn't. No one cared enough to question it.
They subdued the Assassin, hacked him into little tiny pieces. Zexion, they discovered, was very good, almost surgical, at chopping things into little tiny pieces.
"Fuck you, Marluxia," the Schemer said fondly as they tipped out the last chunks of their may-have-been-zombified 'friend' out the upper Window of Many Faceted Glass or Some Shit.
"It appears to me," Luxord offered one pseudo-night, as they all rested for a while. They could still hear their poor troops being devoured. The sound of mouth-less Nobodies shrieking was faintly horrifying. "We are simply feeding our enemy. Perhaps, we should barricade inside and let them starve themselves out."
Food would never be their problem, since they could simply teleport out of this world all together.
"Why don't we just leave all together then?" Demyx wondered. "We could go hang out at Oblivion, we don't really need this place."
Saïx whimpered (a strange and nauseating thing he had taken up doing since they'd killed Xemnas) and Roxas smacked him, reminding him not to fucking do that.
"What about our hearts?" he asked. "Kingdom Hearts has been summoned here. All our equipment and research is here. Even if we did go through the effort of moving our research, what if those zombies then did something to the hearts? We can't move Kingdom Hearts."
The Nobodies grimaced. Of course he had to talk sense, the fucker.
Barricading themselves in and attempting to starve out the zombies worked marginally less well than they had hoped.
The zombies reminded Roxas, personally, of when Axel was in the mind to be an obnoxiously persistent prick.
Larxene was the first to be caught unawares by a zombie who had infiltrated the base. She may have killed the creature, electrocuting it until it was nothing but unrecognizable charred remains, but her right foot was missing and she had zombie germs all over her.
She was effectively euthanized, hacked to useless chunks, and then dribbled out the front door.
This, however satisfying (and practical, Roxas reminded them, forcefully), did not solve their problem. The zombies were getting in somehow.
No one was safe.
Demyx suggested they leave, again and he was smacked for his trouble.
Instead, they decided to have their troops patrol the halls at all times.
For some reason, the zombie horde was not lessening. Occasionally, one could spot them eating each other, but for some reason their numbers were not decreasing.
"Perhaps someone should go investigate where they're coming from," Lexaeus suggested.
Roxas, as he could be the only enforcer to his own threat, watched him warily. He would stand for no backstabbing bullshit while he was still actually depending on these people to help protect the castle.
"Yeah, go ahead, Lex."
The man's brows furrowed at him, but then the giant shrugged and opened a door to darkness.
He did not, in fact, come back.
Well, not alive, at least.
"Damn it," the remaining Nobodies cursed, staring up at the great corpse, which was suddenly very intent upon breaking down their barricade in the name of all zombie-kind.
Lexaeus got Vexen during the fight and then there were eight.
They caught Luxord with his pants down.
Roxas didn't really care to know how the zombies were getting in the pipes, but he could rationalize that once you were dead it wouldn't hurt to break yourself into fitting somewhere.
This, however, was no good. They could not be afraid to use the toilets in their own base.
"Demyx. Do something about it."
"Why me?" Demyx whined, staring with dread at the Toilet of Foreboding Flushing. (Or, that's what Demyx had named it a few seconds earlier.)
Roxas thought perhaps he had acquired a tic in his eye, for it twitched as he said (shouted). "Because you fucking control water and they're in the pipes, Demyx. Shut the fuck up and do it!"
When Demyx cleared the pipes he accidentally shot zombie all over Xaldin, who had, in fact, been in the shower trying to wash zombie guts from his body after successfully exploding one who had been wandering around the halls. He'd hit it just right with a great number of lances… it had been somewhat like resonation and, though an amazing experiment in science, was very messy. Luckily, that zombie goo had been the wet remains and had remained outside of his blood stream.
The arm, which reached through his showerhead and clawed him up while he was still standing poleaxed from surprise, was a somewhat different matter.
And, unluckily (not that Luxord was there to count it any longer), his Nobody buddies weren't taking any chances on him.
"Can we leave now?"
"Now you're just saying it cause I suggested it! If Zexion suggested it you'd listen!"
There was silence.
"Hearts?" Saïx reminded them quietly.
There were murmured grunts of agreement.
"What the fuck!" Demyx protested.
It was Xigbar's own fault really. Zexion had told him the water smelled strange and Demyx had said,
"There's dead shit in it."
But Xigbar hadn't really listened.
Unsure of what would happen, they'd locked him up and watched for the change… When it came… well. You know. They subdued him, minced him, tossed him, like clockwork, really.
Then, after being cooped up in the castle for the past six months Saïx went insane. They sent him out to kill as many zombies as he could while still in that state, but he never came back and they knew that was a very, very bad sign.
The remaining five Nobodies kept watch fearfully (wow, was the siege teaching them all sorts of interesting lessons) for his shambling corpse to come back up the lawn to kill them once and for all.
Their supply of Nobody troops was limitless. But so were the zombies. The whole thing was ridiculous and tiring, Roxas thought, sitting in one of the windows, watching as some of the zombies attempted to crawl up the sheer white face of the castle.
"Roxas!" Axel was in the doorway, looking smug, which was never good.
"What is it, Axel?"
"...fuck off, Axel."
"Saïx took out the back gate!"
Demyx and Zexion appeared as well, looking as if they'd been in rather a rush. Demyx babbled noisily about the whole thing, not that any of them could understand a word besides 'runawayrunawayrunaway'.
"RUN AWAY," he repeated, emphatically.
The three sane members exchanged a glance. Roxas' head drooped against the wall tiredly.
"The hearts?" he asked sarcastically.
"I, personally, am feeling really kind of whatever about them," Axel admitted.
Demyx nodded, emphatically.
Castle Oblivion was very quiet and empty without Vexen, Marluxia, Larxene, and Lexaeus.
"Hey, Naminé?" Axel announced. "I finally brought Roxas to meet you!"
There was no answer.
Zexion turned his nose to the air and said, "She seems to be in Marluxia's old throne room."
"Marluxia had a throne room?" Demyx inquired in awe.
"Why is everything here decorated in roses?" Roxas sneered in disgust.
Axel whispered something into Roxas' ear, which actually made the boy laugh.
So, cautiously, the refugees of the Castle made their way up to Marluxia's old throne room, searching for Oblivion's lone inhabitant.
…the zombie Queen apparently.
Naminé looked quite resplendent in green, really. Brought out the sunken depths of her eyes and all.
"Zexion," Roxas growled. "Why didn't you smell the death before we got up here?"
There was, of course, a perfectly reasonable explanation concerning the fact that they had been absolutely drowning in that smell for the past six months, so that it seemed almost normal, but no one particularly cared. When the zombies advanced upon them to protect their queen, it was Zexion they shoved out front as shield.
Naminé ordered him killed, her chill eyes sweeping over the last three Nobodies.
"Hello, Axel, what took you?" she wondered, irritably.
"Zombie siege, sorry."
"Yes, I know about the zombie siege, I ordered it."
"Oh. Uh. Well. Sorry I missed your… uh coronation?"
Her lip curled at him. "Oh, yes, surely. It was quite grand and regal. What with me being locked in a cage like a planned meal when one of Vexen's experiments got loose."
"Zombies of science!" Demyx squealed. Roxas smacked him and told him to shut the fuck up.
"Sorry about that? Was a whole world away, nothing I could do?" Axel suggested. Roxas, who was closest to him, could feel it as he started to tremble.
"You're going to kill us, aren't you," Roxas noted, tired of playing the bullshit game.
Naminé smiled. "Yeah, probably, take my revenge and all."
"Any way you'd just take Axel and Demyx and let me go?"
She blinked at him in surprise and then giggled.
Zombie Axel kneeled at his feet, pouting, while holding the tray of meat. Roxas ate it at his leisure with his fingers, juice dribbling along the contours of his freshly rotting face.
"Anything else we could get you, my lord, my lady?" Zombie Demyx inquired politely.
"Not now, thank you, Demyx," Naminé smiled and reached out to hold hands with her king.
Roxas rolled his eyes, but obliged her.
"Hey, hey, Roxas," Axel hissed, his vocal cords not working quite the way they used to. "Remember when you said nobody could try anything, or you'd kill them?"
Roxas laughed at Axel's attempt to guilt him. "We're all dead here, Axel. You know, if you stop pissing me off, I'll give you a world, or something, when the horde moves on."
Axel's objections stopped swiftly. "Yes, zombie king."