Once upon a time, there was a faraway land. It is there no longer, and few remain who have seen it, except for one lonely girl who lived through the eons of time to remember it.
Her name was Rue.
Her kind were humans once - or at least, so described what records remained, for she was an archivist back then, tasked with preserving what little knowledge was left of their civilization. Heat, light, and life were all but stories of myth and fantasy, gone for so long that even the eldest hive lords were born trillions of years after the last living cell in the universe shivered and passed away. All that remained were the memories, molecular tales of lives she never lived and stories she never told, tucked away in the little nanomachine "books" that made up her world.
Sunlight. Plants. Laughter. Joy. The patter of feet. The roar of engines. Warmth. Energy. Life
How often she daydreamed, in the slow, methodical way of her kind, of a life beyond the darkness. Though it took centuries for her to complete a single thought in her energy-optimized form, she knew naught else, for time was all she had left - though, as she soon learned, precious little of that. Deeper and deeper yawned the darkness as she and her Remnants dreamed of days long passed, awaiting with bleakest melancholy the heat-death of their universe...
...until one day, a light brighter than all the energy left in the universe combined shone forth into their world.
It was a portal to a parallel universe.
The patter of feet.
The roaring of wind.
Her first step, if it could be called a step, took a century. Her second took almost as long. Her brother latched onto her forecell and led the way, her kin following close behind. As the centuries grew into millennia, and from millennia into eons, the Remnants evolved. At first, even plants were faster than them. Germs and microscopic insects annoyed them. But they were unafraid, for they were immortal; of greater concern was finding their home again after getting excreted from a creature's digestive tract. Numerous times Rue herself had to navigate through the decades to find a signal of her kin after getting eaten and lost by random organisms.
How clearly she remembered the exciting moment she finally retrieved her genetic print from its datacell to finally build her mammalian form...
...and how inexplicably warm was her very first breath.
One by one the Remnants rebuilt themselves in this brave new world. Individuals formed into groups, and groups into communities. Their humanoid forms, they discovered, could change and adapt to this new world with as much versatility as their cellular forms did in the old one. "You are what you eat" proved literally true for them, for they could adopt the forms and functions of the various lives they consumed. Rue, satisfied with her humanoid form, chose to remain pure, opting instead to play a medical- or perhaps better described as technical support- role in the community, since the adoption of foreign genes rarely went smoothly.
No, indeed it did not go smoothly at all, for their greatest obstacle proved to be the native sentients of this land: humans, similar to the Remnants in surprisingly many ways, who did not much appreciate these eerie "imitations" sampling their bodies and killing them in the process. Sparks flew and conflict broke out. As the chief archivist for her people, Rue dug into her libraries and dusted off the lost knowledge of their civilization, valuable information and advanced technologies once irrelevant in a dying universe.
Armed with new and terrible weapons, the Remnants pushed back, their shadowy humanoid visages driving into the natives' settlements and wreaking bloody havoc upon all who resisted. Horrified by all the death she had wrought, Rue identified a man among the enemy who seemed least antagonistic to her overtures and moved to warn him.
Her brother caught her and turned her over to the war council. Judged a traitor, her penance was to be melted away into a crystal core and become a router for the Remnant hive's telepathic communications. Little did they know that her warning to humanity succeeded - or that her contact would have the power to hear her cries.
Once more immersed in the darkness, Rue did as she was told, routing every thought and word in the hive to its every intended recipient - plus a few more they did not intend. The humans knew not whence these mysterious signals came, and initially not even what they were until a few intrepid mages realized they were spoken language. Armed with insider knowledge of the hive's movements, an elite team moved in to destroy the router core and cripple the invaders' communications.
It was only the foresight of their leader, the man she'd contacted from the first, that prevented them from killing her. Using a spell she communicated to them in the weeks before, he reconstituted her humanoid form from its prison, cradling her shivering body in his arms, before destroying the rest of the crystal core.
The war ended shortly thereafter. Having studied the weapons and technology of the Remnants (or Voidspawn as they came to be known), humanity finally pushed back against the invaders and drove them into hiding. The enemy was not as immortal as they once believed; there were limits to their regenerative powers, and humanity tested them to the breaking point. For the first time in untold eons, Remnant lives were lost. Shattered and broken by their defeat, they retreated into the shadows of the world to nurse their wounds...but never forgetting the shame inflicted upon them.
Rue, however, married the man who rescued her, and for the first time in all her millennia of life, discovered the joys of family. Children, grandchildren, and great-grandchildren sired under her name, and when her first husband breathed his last in her arms, she remarried and lived happily until he, too, passed away of old age. By the time she found herself alone once more, reminiscing about times past amongst the lively chatter of her descendants in the comfort of a Centerville tavern, her dark past, once thought to be gone forever, returned to haunt her.
"The beast has attacked again! Are any of you adventurers? We need your help! Come with me to the mayor's office, he'll give you a good reward for your help."
When she picked up her magic staff to answer the call, she did not know that the beast in question was a being she dearly wished would have stayed hidden: a Remnant, back for revenge, and an omen of threats to come against the humans she had come to know and love.You shall only touch them over my dead body.