Picks up sometime post- Issue 3 of the Spike comics (aka post-Angel/ BtVS), then travels back in time to Buffy Season 5.
In The Space Between
Chapter 1: Bitter Beginnings
Spike stared silently across the worn-out battleground this section of Los Angeles had become since that supposed final battle. His eyes were hard, shining with the trademark thirst for violence that had always been his. His body betrayed a certain hardness to it, a product of weeks or months, or maybe it had even been years by now, of the cruelty of this place. He cocked his head suddenly. There were demons nearby. And they were desperately in need of an ass kicking. They just, perhaps, didn't know it yet.
"Spike?" The voice behind him was soft, trembling in the stale air. "Spike, where are we? Why is it so cold?"
Spike sighed slowly, the hard shine of his eyes fading rapidly into a tired gaze that spoke of loss and frustration. The hardness drained slightly from his body, lending a decidedly worn out cast to his figure. He turned his head, "Fred." He said softly, his voice pained. "Of all the times…" He swung his gaze back around in time to see the first of a group of green-scaled demons round the corner of a slowly collapsing building. Grabbing Fred, he darted down an alleyway filled with rubble and dust. "Just be quiet, alright Love?" He murmured softly into her ear while pressing her into an alcove between a rusting dumpster and a mostly-intact wall. "Just stay here."
"And what'sss thisss?" A reptilian voice inquired from just behind Spike. "Tryingsss to protectsss your girlfriend?" The voice continued, its owner advancing steadily into the alleyway. "It'sss pointlessss." The evil snake-like voice spoke of petty victories in back-alleys, but also a certain pathetic desperation. The humans were beginning to either run out or wise up, and the world was suddenly becoming tougher again. There were few easy kills these days, and the thought of just one more had the scaly demon salivating already.
Spike turned slowly, letting a particularly menacing gaze creep across his features, even as he let his vampire features slip across his face. The green-scaled lizard demon looked slightly taken aback for the briefest of seconds as his dreams of an easy kill dissipated. "Vampiresss?" It commented, its long tongue flicking rapidly in and out of the air, tasting the molecules still suspended. "No… just the onesss." It concluded, "Andsss a tasssty meal." Its reptilian eyes blinked briefly, as it calculated the odds. Killing one vampire wasn't so difficult, if it meant an easy kill to follow. "Ssso, we fightsss for the girl?" It finished, its eyes growing wider as several of his fellow demons joined his side. Things were looking up. "The oddssss aren't in your favoursss."
Spike managed to control himself long enough to roll his eyes and shrug his shoulders. "So it won't be easy, mate," he began, throwing himself towards the lizard's companion on his left, "Just as well I've been itching for a spot of violence." His eyes glowed golden as he snapped the lesser demon's neck easily. He flashed the talkative leader a toothy smile. "S'no fun when the odds are even." The cocky show he put on was almost as old as he was, and he slipped into the role with ease, even as the weariness persisted just below the surface. He shrugged it back, one more fight for survival. One more notch on the old sword. Figuratively, of course. Only the ponce himself would walk around toting a sword.
No sooner had the lizard demon blinked than another of his companions was little more than a crumbled heap on the dirty ground. "My brothersss…" It hissed nastily as it threw himself at Spike, teeth and claws bared. Losing the kill had always been possible, but to lose his brothers, just when their numbers were dwindling already… "Getsss the girlsss." He tossed the order back to his remaining relation, spite and anger tangibly drifting off him. For long moment the second lizard stared at the wicked fight going on between his eldest brother and the vampire with the flashing golden eyes and black leather coat. Then his gaze fell upon the tiny brunette hiding as best she could in the back of the alleyway. A ruthless smile closed upon the lizard's face, revealing wickedly sharp teeth and a forked tongue that tasted the air just as his brother had earlier.
Fred backed into the small space as far as she could fit; fervently wishing that she had some sort of power… In the space between two moments, Fred was gone, replaced with Illyria. To be more accurate, the lingering conciousness that had once been Fred was pushed back down into the depths of cellular memory as the body's invader took active control. Humans proved to be a truly tenacious sort of conciousness that strove to continue to exist, the ancient demon god had discovered. Illyria truly appreciated the difficulties faced by her vampire companion on that front.
The approaching lizard blinked rapidly as the cowering girl stood up, suddenly clad in a red bodysuit, with blue locks drifting about her blue-tinged face. The scent of scared girl still hung in the air, but was rapidly being replaced with one of ancient and dark power and strength. The reptile stood frozen as his brother remained locked in battle with the vampire behind him. "So," the being before him said slowly, her eyes locking with his own, "You wish to eat me?" A short sharp laugh filled the air. "I've picked my teeth with the bones of more worthy opponents than you."
"Illyria!" Spike's voice cut out before the sound of the lead lizard's tail impacting with his chest halted his breath. Illyria watched disinterestedly as Spike pulled himself out of a pile of broken concrete. "A little help here?" He said finally, noticing her stare and the frozen lizard before her.
The demon goddess stared at Spike for a long moment. "Very well." She murmured, the disinterested look never leaving her face. Seconds later the frozen lizard would never move again and the lead lizard suddenly found himself caught between the vampire he thought he'd nearly bested and a being that smelt of stronger magic than he had ever tasted. And the corpses of his three brothers. A bitter sneer crossed his reptilian face as he hid the sharp sorrow of the loss from his opponents and muttered something just under his breath.
Very suddenly, he had disappeared. Along with the dust and the dirty alleyway and the tired battle ground of Los Angeles. Spike stared up at the stars and the darkened evening sky that had replaced the garish oranges and yellows he had grown used to. "Don't rightly think we're in Kansas anymore, Love." He muttered, thrusting a hand into his coat pocket instinctively, even as he remembered that he'd had no cigarettes for weeks upon weeks now. Gazing about the trees that surrounded him, Spike frowned, his forehead furrowing slightly. He took a deep breath of the still night air, something akin to familiarity tickling his senses. "It don't… it can't…"
"We were never in Kansas." Illyria replied, her voice icy. "We have, however, left the dimension we were residing in." She stared at her new surroundings with mild distain and began walking. "Come halfbreed." She commanded. "I am your priority."
Spike followed just a step or two behind, gazing at his surroundings as strange expressions flew across his face. "It's all familiar," He muttered darkly, "But it can't be." A worried expression finally settled across his features, "Blue, do you have any idea…" But the words died on his lips as a figure stepped directly into their path.
Spike stared for a long moment at the creature before him, who like him, seemed to be frozen in shock. "Bloody hell." The two blondes exclaimed, their identical accents drifting into the evening air as Illyria looked on, an expression of utter boredom across her blue-tinted face.