Near the ancient city of Gadobhra, just outside its walls, was a small cave that hid a newly created passage to the lands ruled by the Fae. To a human, it was well hidden. The ancient druidic magic that caused the rapid growth of the forests was still strong, and the woods here were a dense tangle of trees and heavy underbrush. The lumberjacks who harvested the trees could make no headway against the forest, and only the well-built road leading to the city was kept clear. Assuming you knew it existed, it would require using the bed of a small stream as your path to get to the gateway. The trees grew thick above the stream, nearly to the water, and staying in the stream was difficult. A traveler would be soaked to the waist for sure and, in all likelihood, to their eyebrows when they slipped on the slimy rocks and went under in the deeper pools.
And it wasn't very nice water to wade in. The spring it came from was in the Beast Woods but too near to the slaughtering pens. The murky water tasted foul from the blood that soaked into the ground. It was an unpleasant experience, and the owner of the gateway felt it was well hidden. And it was...from humans. If you were a hungry sedge beast, it looked like the entrance to an all-you-can-eat buffet. Like the fast-growing forest, the ancient druids had created the sedge beasts. Their job was to cleanse the land of the dark magic that seeped from the dungeons in the city and prevent the city from expanding. The horribly mutated cattle had a taste for magic, and the sedge grass was their choice of meal. The sedge absorbed the dark mana; the beasts ate and processed it, releasing the raw mana into the surroundings minus the darker parts.
Sedge beasts bred amazingly fast, growing to adulthood in a month and carrying calves to term in only three weeks. If their numbers dwindled, that time went down. No pack of wolves or an army of butchers could destroy all of the herds around the city. The Butcher was trying his best but only increased the breeding rate. He was doing his part to disperse the dark mana by curing the meat so humans could eat it. There was a strange symbiosis between all of them. The dungeons pushed out rivers of dark mana controlled by the forests and absorbed by the sedge. The beasts ate the sedge, and the Butcher got rid of the beasts, or they died of natural causes when they were a year old. (Except for a few that led the herds, growing stronger and meaner as time went on. It was an old superstition that you should never name a sedge beast or make a pet of one.) The mana released was absorbed by the druidic spells still active after centuries and was responsible for the rapid growth of woods, sedge, and beasts.
Only Baron William's determined efforts to make a few dollars for his overlords at ACME changed the status quo. Dark mana had flowed like a river, creating daemons from the raw materials it encountered, warped the wildlife, and mutated some of the plants. This was solved when a Barmaid tapped into the flow of mana and used it to brew beer, dispersing the darkness across the land where it was processed in the bellies of thirsty beer drinkers.
The only problem was the sedge beasts. They had been bred with a need to seek out dark mana and eat it. There was less mana flowing from the city, and the sedge had lost some of its flavor. The beasts pushed into the forests, looking for tasty sedge grass that still had a good flow of bad magics adding to the taste. The calves were the worst, causing the Shepherd no end of trouble as they got themselves lost in the tangled forests. It happened that on one day, three such adventurous calves found the stream and drank from it. Where anything else would have avoided the water, they found it tasty. There was something about it that hadn't been there before. They explored upriver, swimming when they had to and munching on the sedge they found along the banks. Eventually, they came to a cave. Most herd animals will not venture into dark caves thick with the scent of blood. Sedge beasts didn't care. They lived fast and died hard. And knew that most predators would die shortly after eating them. The cave beckoned, and the three calves wandered in and found themselves someplace else.
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Silverthorn Vale wasn't a nice place. Dark mana laced with poison had seeped into every bit of it. All the flora and fauna were poisonous, and most had deadly fangs or thorns to ensure any creature got a good dose if they weren't careful. The first little calf frolicked in the meadows, eating the long grass with sharp leaves. Imbued still with large amounts of poisonous mana, the new grass was a welcome change from sedge grass. The calf ate, pooped, and repeated the process many times as it wandered, leaving ill-smelling piles of dung, which anyone who has been to a pasture knows to avoid. Eventually, it was found by a hungry dire lizard and eaten in three large bites. This went poorly for the dire lizard, for while the sedge calf was very tasty, it was also quite tough with multiple horns and hooves that didn't like being digested. It took a week for the poor lizard to get over its meal, at which time, being quite stupid, it went looking for more.
The second calf quickly found a large mound of Silverthorne vines. The vines were too tough for it to eat, and after getting a dozen cuts, it stuck to eating the berries, pushing further and further into the bushes in search of the fruit, finally becoming quite stuck. It had also eaten enough poison to kill a dozen manticores. Where it died and decayed, sedge grass started to grow, the seeds coming from its stomach and digestive tract. Likewise, wherever the first one had stopped to poop, the grass began to grow, absorbing the poisonous mana from the ground and flourishing.
The third calf ate its fill of the grass, dumped a few loads of fertilizer, and became thirsty. It went back through the gate, and while getting a drink, it was swept downstream to a spot outside of the woods and eventually made it home to its mother. During the next two weeks, the calf grew to maturity but ate poorly, missing the poisonous grass of the vale. It decided to return for a meal, with several of the herd following along. It had complained so much about missing the taste of the sharp grass that everyone wanted to try it. The pack of seven mostly grown sedge beasts followed their leader and disappeared into the cave.
What greeted them was a small meadow of sedge grass that was quickly pushing out the native sharp grass, giving the Fae their first lesson about invasive species if anyone had been around to notice. The sedge grass on this side of the gateway was much tastier, having drawn both dark and poisonous mana from the soil. The sedge beasts got to work doing what they do best: Eating, pooping, and breeding more sedge beasts. The ancient druids would be proud to see how their work was being carried on without them.
The dire lizards were thrilled to have more of the tasty new prey animal. They were less thrilled to find out that they might be the prey animals as well. While one calf was a small snack, full-grown sedge beasts could give them trouble, and a herd working together was death beneath trampling hooves or from impaling horns. Sedge beasts were also omnivores and not averse to adding a little meat to their diet. The two populations of animals were equally matched, and while they occasionally met up to see which species were on the menu, they mostly avoided each other. The snakes and other fauna were upset to find out they now had to worry about two sets of predators, and the flora was adjusting to providing dinner for the ugly cattle. Silverthorne vines took the lives of many sedge beasts, their poison enough to overcome even an adult that ate too many berries. However, the sedge beasts were adapting generation by generation.
When Lord Alwyn got around to exploring the vale and making plans to begin caring for it, he found his work already started. Several acres of sharp, poisonous grass had been converted to sedge grass. Silverthorn enclosed the area, but the sedge beasts were pushing it back bit by bit as the herd grew larger. Only a few more beasts had made their way here, but with their rapid growth, over two score cattle were grazing in the toxic meadows.
Alwyn turned to Twitterberry, "Ride to my herd master and inform him that we won't be bringing the unicorns in to begin the cleansing. The horn-ponies didn't want to come anyway and I've found a replacement for them."