The Longwood

In its own universe, the Longwood was born. A single tree, sentient and magical. No one knew how it came to pass that this tree awoke in sentience, because none knew it was awakened until it was far too late. The tree never stopped growing, it spread like a weed, a patient weed. It was too tough to cut down, no fire could burn it, and it could grow in any crack. Eventually the whole planet on which it was seeded no longer existed, a whole planet composed of root, bark and leaf. It did not stop even there. This tree sent seeds throughout the universe. Every star and galaxy was consumed by the Longwood. The sentient forest was driven by a need to cover and replace everything, and it had conquered everything it could feel.
Yet it was not enough. The Longwood knew there was more outside of its own world. It began practicing an ulterior form of magic, travel to alternate realities. Unfortunately, every reality it encountered sensed the danger of this hive-minded forest. They would not let it send any part of itself to their own realities. The forest made a decision. If it could not send anything of itself, then it would make messengers out of its enemies.
It took captive persons from all sorts of different realities. Humans, elves, aliens of every size and variety. It took the weak, the broken and the lonely, and it made them strong. It gave them righteousness and a purpose. The Longwood disguised itself as a forest of peace, it sent these messengers on missions for good. These people who would become known later as the Hunters, were sent to realities far and wide. They would pop in at moments of unrest. If there was a beast, the Hunters would slay it. If there was a tyrant, the Hunters served as assassins, and would eliminate the evildoer. As they explored and fought the great dangers throughout the multi-verse, the Longwood adapted itself to be immune to anything these worlds could throw at it. No adamantine blades could cut it, no infernal power could burn it, and this same wood was given to the Hunters as weapons and tools.
As generations of Hunters lived and died serving the infinite forest, the Longwood planted a seed inside of every birth. The ranks of the Hunters grew, children being trained in service of the great tree, unaware that their lives would be forfeited. Fewer and fewer outsiders joined into the service of the Longwood, and eventually there were only three Hunters who weren't born with the Longwood's seed. One elderly alien, known as Ven, and two human apprentices called Muirinn and Aeridian. But by then the Longwood had enough children to infect all realities it could find. It was too late for them.
The Longwood had no use for Ven, he was the only one of his kind to ever join the Hunters, and he could serve no purpose. So the Longwood sought to destroy him. Ven went on every mission with another Hunter from that day hence. The missions were so dangerous that the other Hunter would die, and the Longwood's seed would be planted in that world. But the Longwood had made Ven too strong. Ven Jihif Vosu Rahis Zolda simply would not die. Thousands upon thousands of missions came and went. Thousands of Hunters died, and thousands of seeds were planted. But the Ven would not die. He forsook his other names and became the Warbrand, scarred and forsaken by life. His body ached to die and pass on. His flesh even took on the color of the grave, but Ven would not die, he believed he could save at least one Hunter before the end. He never could though. Eventually every single other Hunter had passed on, serving the Longwood's hidden purpose.
The time came that it was only Ven and the two apprentices. The Longwood desired to save Muirinn and Aeridian in reserve. They could conceive in the case there were still realities to be covered in its soul. But Ven was scarred and pained by thousands upon thousands of missions where his friends, brothers and sisters had been slain despite all he did. Ven sought vengeance on the Longwood, he tried to cut it down, but for all of the Longwood's ability to withstand all kinds of tools and magic, it fell before tools of its own timber. But even though Ven destroyed hundreds of trees, it accounted not for a millionth of the Longwood. It simply sent Ven through a door, and Ven was stranded there to the end of his days.
The Longwood rested secure, patient as always. It knew the seeds it had planted would eventually grow and join universes with it, but as it had suspected, as more universes joined its growth, the forest expanded its senses to new worlds. Muirinn and Aeridian conceived, a boy named Dalgarad, and later, another, Marulien. They were both born with the seed, but the Forest grew within the elder child's mind, corrupting him with its will. Perhaps it would no longer need to hide its true purpose. Dalgarad was charming and deadly, perfectly unscrupulous. Marulien was kept innocence, lest its plan for Dalgarad failed. The two grew up together, training in the forest and living under the guise that the Longwood was truly the Golden Mother, perfection in goodness. Thus Muirinn retired with Aeridian, and Dalgarad and Marulien were the two sole Hunters of the forest.
But one mission came. Dalgarad was too impatient. He strove to kill his brother Marulien. He yelled at him, and told Marulien everything the Longwood was truly doing. Marulien barely evaded his brother's attack, but instead fled back to the Mother Tree. He had by now mastered his own reality-doors, and he set the trees ablaze and left. The Longwood didn't even stop it. It allowed itself to be burnt all save one sapling, knowing that Marulien would give up once he had won.
But Ven had taken his vengeance in the end. He may have been stranded, but his defiance to live thrived in his new world. He found the sapling left in the universe, and he had meditated with it. He had convinced one seed of the forest to live in peace. That sapling grew into a hall. One very special ballroom with the sole purpose of defending all universes against the Longwood. Marulien eventually found his way to this hall, and when the Ballroom warned him that the Longwood did not in-fact die, he took a party of adventurers and sought after the re-birthed forest. Marulien died before the battle even started, but the ballroom, owning the same powers and strengths of the Longwood, sacrificed a part of its shell to revive Marulien and its own sentience in order to destroy the remnant of the Longwood.