The Wolfsbane stopped working.
It had only been Severus' quick casting that stopped Remus from harming him. He never thought he'd see the monster again. Old nightmares came back.
He blames himself, for what happened after.
Remus noticed immediately. Severus always looked like he was holding his breath. He flinched when they touched. They still lived together, they still slept together, in all appearances it looked like they were still committed, together. Severus made tea and Remus put out biscuits. Remus fed the cat and Severus weeded the garden. Severus repaired books and Remus did the dishes. They had lived like this for nine years, comfortable and quiet. Remus had a job at the local pet store. Severus did not teach. Severus assumed that this was like the tide, memories flowed in and out and always passed. They had lived through so much, he had no reason to assume anything would be different.
They went to Poppy, but Severus ran his own diagnostic tests. He brought out his cauldrons again, rusted and decayed from disuse. They could rework the formula, again. They would find a solution. It had been years since he'd opened a potions manual, but in this he was committed. He didn't clench his teeth when dogs passed, he didn't draw his wand near the moon. They made love. They cast wards they never thought they'd use again. Severus promised to be by his side when he woke, potions prepared and ready. This was like the tide, ebbing and flowing.
Remus must have seen something different in the red lines of the diagnostics. He doesn't know how to explain otherwise.
Because after six more moons without Wolfsbane, Remus bought silver shackles and silver darts. Severus was furious. His hands shook making tea and the biscuits Remus set out tasted like ash. The cat hissed whenever Remus tried to come near and the dishes showed fractures from Reparo. Their tenth anniversary fell on the seventh moon and Severus stood at the door, all night with his wand out reliving the Tunnel and the Willow and the Shack, enraged. The rage spent itself when the wards dimmed. Severus poured balms and salves and potions everywhere, feeling that with each, he renewed his commitment: I love you I love you I love you and no despair. He'd had fears in the Dark days. He hadn't wanted to fear again.
Remus endured another six moons, in the process losing his job at the pet store because the animals were terrified of him. He got another delivering packages, working night shift scanning parcels and express delivery post. Severus worked furiously on the Wolfsbane, ignoring all the data that didn't tell him what he wanted to see. Minerva stopped by to pick up their cat. She took in the state of the gardens—weeds everywhere and rosemary growing out of control—and held her tongue. Severus never took such offers kindly. It was the tide. Nine years, their life had been roses. Twelve years he'd waited for the Dark Lord. One miserable year when everyone had thought him a traitor. It was the tide.
The tide that brought Remus home at four in the morning, exhausted. The tide that made his joints stiff. The tide that made him snap, once or twice, in anger. The tide that made his body tremble. Severus told himself. But he knew. The tide that came would have to go back. That was always its promise.
After, after, Severus finds the files, hidden in a bag of cat litter. It's a thick blue folder with "St. Mungo's Werewolf Ward" neatly inked on it.
That was always its promise, but Remus saw something different. He must have quit his job, or been fired after his joint pains grew too severe. He never told Severus. He couldn't bring himself to, perhaps, Severus thinks. He was trying to protect me, perhaps. But he knows the truth. This was Remus, saying nothing, doing nothing. This was what he wanted.
The fire was horrible, but only a few students at Hogwarts happened to see it. The people in Hogsmeade warded off ghosts and put up silencing charms. Minerva slept through it—she'd had an exhausting day.
He blames himself. They found no bones. Remus collapsed the tunnel under the Willow. Hagrid found a silver knife and placed it in Severus' open hands. Severus knelt on that ground, still warm, until the sun set.
The tide.
He watches the tide come in and out, here on the coast. Sometimes he fancies he can feel Remus next to him, but he knows it's a fancy. Dreams, and nightmares, still come and go. It hasn't changed. One night he threw the blue folder into the black waters. The next morning the papers were scattered along the shore, words washed out and stuck with seaweed. He picks out the dim echoes of the ink—degenerative—and walks on.
His body washes up with the tide, unrecognizable, a week later.