Derek starts counting down the days when he first hears the telltale stutter in his mate's chest. Out of inevitability, not morbid fascination or anticipation. Derek would never want Stiles to go. If he could have his way, he'd lie down with him and die like so many romantic fairy tales. But Stiles would never stand for that. "You are such an idiot." he'd say fondly, laugh lines decorating his face like the millions of moles Derek has yet to finish counting.
So he counts every beat of his partner's heart, every skip. He prepares for the day they have to part ways, until inevitability catches up with him, too.
Despite the six years advantage the werewolf has, stress of living with supernatural beings catches up to humans much faster. Stiles was, is always strong. His body disagrees at times.
If they lie closer every night, neither cares enough to mention.
One night, Derek is roused by Stiles moving off the bed. His mate grips his hand in reassurance. "Go back to sleep, hon." he purrs, and Derek does just that.
The next day is like every other. They wake up together at dawn, coddled together in the blankets and enjoying the closeness. Stiles gets up first to make breakfast for the pack staying in the house. Derek showers and trots downstairs, drawn to the kitchen, to wrap his arms around the younger man's waist and bury his nose in the crook of his neck.
"That isn't very conducive to the making of bacon, Derek." Stiles teases, leaning into the embrace. Derek just hums, losing himself in the smell for a moment.
When he pulls away his hands linger, but business can't be ignored. The sooner he deals with it, the sooner he has time for his husband. There are finances for the construction company they run to cover the pack, requests from other wolves to visit his territory. Not many get turned away anymore, not since Beacon Hills became a sort of safe haven for his kind.
Breakfast is called and the pack descends upon it, glad for an easy meal and each others' company. The younger ones are rowdy, but put in their place fast, and Stiles is smiling all throughout. Derek shares the happiness as well, Stiles' grin contagious.
The crowd does disperse eventually. Derek helps Stiles with the dishes, and the rest if the day is spent together, wandering the woods, enjoying the sunshine, teaching the new pups that stop by after school. Scott and Allison's grandson never stops moving.
Dinner is a huge affair. Twice a week the entire pack gathers, brings a portion of food to share along with their stories from the days they spent apart. The large house is filled with laughter and family. There are no more tense meetings that felt like preparation for war, no unknowable threats bearing down on them. It's the bliss Derek remembers from a lifetime ago, before a woman burnt down his life and a boy saved him from downing.
That night, when the house is settled and Stiles is curled up within Derek's limbs, Stiles whispers his name. He opens his eyes, meeting those deep caramel brown eyes he could get lost in for days (and the first time he said that aloud, Stiles told him that was the most cliched thing he ever heard, but kissed him for it anyway). "I'll always love you." he says, a weight behind the words. Derek smiles, brushing his fingers through Stiles' grey hair. "I know."
Stiles laughs at that, shaking his head. "Thanks for that, you scruffy-looking nerf herder." Derek chuckles, leaning in for a sweet and easy kiss.
"I'll always love you, too." he says eventually, once they break apart. Satisfied, Stiles hums and pulls himself closer, falling asleep not long after. Derek follows, warm and content.
The sky has barely started to lighten when Derek is startled awake. Something is missing. He listens, but the room is quiet. There are the faint sounds of the pack scattered through the home, and the winds and birds outside, but his bedroom is quiet.
His mate is quiet. His chest doesn't rise and fall. His face doesn't twitch with dreams. His heart doesn't beat.
Stiles is dead.
Derek can't help the broken, heart-wrenching howl that rips from his lungs. When he stops, his chest heaving (like Stiles' used to, panicked or aroused or bursting with laughter) he clings to the body that once held his heart until the sun is over the trees.
Isaac gently pulls them apart, Derek resisting only slightly, and sit his Alpha in the chair by the wide window overlooking the forest. But Derek doesn't watch what happens next, when someone removes Stiles' body oh so gently, taking his scent and the slow stench of death away.
The lone Alpha stands after countless time passes, rifling through their (his) closet to find that ratty, old, red hoodie. He takes it to their (his) bed, and folds in on it. Inevitably his tears run dry, and the hoodie doesn't smell like anyone anymore.