Disclaimer: I don't own or claim to own anything to do with Joss Whedons Buffy/Angelverse or Gamesworkshop's Warhammer 40K. That is all. Enjoy!
Angel flew into the wall. The costume possessing the boy, wile absolutely awful at fighting, was stronger than the average fledgling, he couldn’t afford to keep kid gloves on, it would just be another awful thing to add to the atrocities. He bit the creature. Then spat the blood out, it tasted like plants? The green monster picked him up, lifted him above its hunched head, and threw him down the alley. He picked up a pipe and threw it at the creature; it pierced through its forehead, just. He ran down the alley past the corpse, he needed to get to Buffy and make sure she was safe. A groan stopped him in his tracks.
It was still alive!? How! The monster slowly pulled itself to its feet and threw itself on him, choking him. He threw it off then snapped its neck, it continued to groan. He grabbed hold of the head and pulled it off. The head tried to bite his arm fro a few moments, and then died. It changed back into the head of an adolescent human boy. He dropped it, sickened. For a moment his nose had been overwhelmed by a foul smell, like a huge compost heap full of fungus. But he couldn’t smell anything now. He ran off to the summer’s house.
The spores flew on the wind; the pervasive energies of the Hellmouth entered and changed them, increasing their all-ready deadly potential. The spore’s possible genetic structures morphed by the demonic energies, the binding-algae cell structure becoming stronger and holding more potential than it did. The creatures that could grow from them changing...
The spores themselves became stronger, tough, much more fireproof and hardier, with the ability to grow much faster using less nutrients and become much larger in more than one direction. Within a few months the plant would start to constantly produce more of the spores and ensure the deadly cycle would continue and increase. The potential to become one of the deadly possible creatures of the cycle increased and the spores would be able survive in many more conditions. The energies withdrew, their job finished.
The spores flew on the wind until they reached forests. There, they took root quickly and strongly. Wildlife migrated away, aware only that they couldn’t find any food there. Soon new creatures, monsters emerged. Brightly coloured voracious pig-like creatures emerged, preying on the undergrowth and ripping undergrowth apart. Soon small green monsters emerged, fighting and biting everything.
Then, marginally larger monsters emerged, capturing the pig-creatures and slaughtering them. Tearing down branches, gathering them up, finding anything of use and then bringing it back and hunting smaller wildlife while the smaller ones scrabbled in the soil for beetles and bugs to munch.
Finally, the large ones emerge. Hulking green monsters, with scrunched brows, large noses, no ears, red eyes and bulking muscles. These fought, yelling and laughing in their rough tongue. Killing and slaughtering the smaller ones on whim. Tearing the pig-creatures apart and ignoring the smallest greenskins. Trees were pulled down, torn apart and shoved together into crude huts. Wildlife was run down and pulled apart, skinned and eaten. Bones ground into crude but sharp knives with their teeth. Skins stitched together for rag like clothing.
Then, they multiplied, small groups wandering off and making small camps. The opened nutrient pods closed and began to grow into trees. Using an accelerated and adjusted form of photosynthesis with strong, supported large leaves in order to generate massive amounts of energy-rich nutrients and pumping them into the soil.
Then a new monster emerged, slightly larger than the rest, more violent but more intelligent. It grew quickly, fighting the others of its camp and putting on more height and muscle, getting stronger and stronger. It beat the members of its own tribe into submission and led the way into the forest. They found then nearest camp and the leader tore the other leader apart with its hand. The new ones joined, starting the cycle. Soon all the tribes were united, while more monsters continued to grow. The spores being spread after each fight that ended with a death, in other words, nearly all.
Then news was spread. A lone monster had wandered off on its own, desiring to see more than the large forest. It walked to the edge then watched the nearby town of Sunnydale with beady eyes. It heard the sounds and sights and smaller pink fleshy things and took the news back. The Boss smiled, he had enemies. Good.