Part I : things fall apart
Sid despaired of ever being able to explain to Ellie. They were too young to understand. So she simply repeated what she'd always said, that there wouldn't be much more time, and hoped that when Ellie was older, they'd think back and it would all make sense to them in retrospect. She wished Ellie had other people to lean on. It was tough being your younger sibling's entire support system even without knowing you were going to die before they were an adult.
At least Ellie would have a place. She'd made sure of that, teaching them all the mapping skills and techniques she'd developed over the years, things she'd learned from the spirits about how to see not only what a place was like right now, but what future transformations it held within it. The Staircase had potential under the surface everywhere, and Vigmar, Sid's special friend among the spirits, had opened her eyes to the possibilities.
Her head ached with an intensity that made her want to do nothing but sleep, but Sid couldn't afford to waste one of her precious few remaining days. She didn't trust herself to navigate the terrain alone, so Ellie had to come along.
Sid stumbled at exactly the wrong moment. One of Vigmar's cubs startled her, she looked the wrong direction, and lost her balance. On impact she broke through the thin crust of the world. Ellie looked down from the edge of the hole she'd made, falling. Sid yelled up at them, "Stay there!" She groped at her ankle. Not broken, she was sure. Pretty sure.
Vigmar appeared in a swirl of smoke. Seeing the angular fox-face, the white fur-like tendrils, Sid thought again how beautiful the form was that her friend had chosen. The ears twitched as Vigmar looked up to where Sid had fallen from. The nose pressed into Sid's hand, then explored her ankle, which had begun to swell.
A cloud of Vigmar's substance began to wrap around Sid. She slid her hand along the solid part of the flank to where the substance of her friend thinned to fog's cool cloudiness. Her fingers spread within the fog and they were linked, as though it were only a moment since the last time.
The chill spread into Sid's limbs. Vigmar showed her two choices; neither was very good, Sid thought, but they were both better than any alternative the spirit knew of. First option, Vigmar could use a great deal of saved energy to repair Sid enough to allow her to climb back up to Ellie and return home with them. But this would drain the power Vigmar had been collecting to join with Sid when Sid's short remaining time ran out. It would mean Sid would die, and Vigmar would be alone again.
The other option was for Sid to join with Vigmar now. The energy was not quite enough, not alone, but neither was Sid quite dying. Sid's remaining energy would suffice if she let Vigmar take it, and then the two of them could become one, as they had been planning. But that meant Ellie having to find their way back home alone, meant Ellie losing Sid many weeks earlier than Sid had thought it would be.
Sid had done everything she could for Ellie. She couldn't bring herself to give up Vigmar, too. What she'd taught would have to be enough. She showed Vigmar her choice, and the spirit swirled in closer to her, began to pull the energies from her to fuel their union.
It seemed moments later and a long time, too, when she heard the screaming. Ellie was screaming. Sid tried without thinking it through to go to her, and it was Vigmar's head, the ghostly fox-like face, that rose up to Ellie. "Go home where it's safe," she said, not able to think clearly enough to realize that Ellie might not understand it was Sid saying this, might not have any idea what was happening beyond their beloved sister not coming home with them this time.
The pain when the sword stabbed through her, through the spirit's side, opening a vast wound, didn't make it any easier to think. The entity that was now Sid and Vigmar screamed and curled into the center of the clearing. Vigmar's children and other, smaller spirits with less mind and more motion crowded out to the defense of their mother, their great one.
Sid didn't even see whether Ellie escaped.
Part II. putting the pieces back together
Jules was the first human Sidmar had met since her sibling had given her the wound. She had searched for the younger sibling, had sent sprites scuttling after them, but they had departed into space, out of reach, too quickly. The defenses the others had made for her kept the rest of the humans away. The others had feared that the humans might try to finish what the one had started.
But then one of her children found Jules, and smelled Ellie, knew Jules had been with Ellie, and Jules had followed that child to Sidmar. She had even given the child a name. Marshmallow, a name that reminded Sidmar's human memories of camp fires and sweet treats, just the right name for this sweet flame of a child, she thought. Sidmar wondered why she had never thought to name her children. It was because they had existed before she'd had language, she told herself. Before Sid became part of her, using symbol-sounds to encode and communicate her thoughts hadn't been something that had occurred to her to do.
The wound had never healed, but Sidmar had got used to it. It had slowly worsened as the energies she used to fix the damage it kept doing to her coherence dwindled, as she found it more and more difficult to draw energies from the world to refill her stores because of the barriers protecting this clearing from invasion. To repair the wound itself was impossible; it had been given by a sibling, and that sibling had never given the respite to allow it to heal. The sibling had instead departed, going far away. But they had finally returned.
Sidmar urged Marshmallow into action and fed her child the energies it would need to carry Jules back to her family before the poisons she'd ingested defeated her resistance. She wanted to ask Jules about Ellie. She could see them in Jules's mind, a shape called Elliot, so they had stopped using the nickname, it seemed, even though it felt like Jules and Elliot were good friends.
Marshmallow left with Jules. Soon after that, there was a change in the wound, one Sidmar had given up on ever feeling. It was difficult to wrap the human language around this; requital was the word she had settled on. The wound, unrequited, would never have been able to heal. Now it could.
While it was reason for joy, this new and unforeseen possibility of healing, the human part of Sid did not feel unalloyed happiness. The requital meant that somehow, her sibling, the one who had inflicted the wound on Sidmar, had been wounded similarly. The similarity had to be strong, but it was unclear how it had come about. And it might kill them both now, if Elliot died of it. As a human well might.
The spirit fox half of her didn't understand how it was possible, but Sidmar felt that if that happened, she would mourn her younger sibling's death more than her own.
Over the next period of time, Sidmar rejoiced to know that Elliot was healing, as she finally felt her wound close, and was able to stand and play again with her children. She began to negotiate with the small spirits that inhabited the clearing, coaxing them to greater agreeability as she regained her original mastery over them, as they returned to being her attendants as they once had been.
When she was fully healed, she had the energy to spare to create a message that she sent to the Hills with Marshmallow as the messenger. "Let us discuss how things might change for the better," it said, "if humans no longer wish to threaten us. Let us discuss how we have finally found a path that may safely lead through the dangers of the world."