He placed the Cup in the vault that evening.
He went down alone — the vault was under his Fidelius and no one else but family could find it without him, which meant the trip was always his alone. He set the containment box on the shelf beside the locket and looked at the two objects for a moment.
The locket and diadem: destroyed Horcruxes, the object remaining. A record of what he had done in third year in a cold room in the Room of Requirements with a goblin knife and the specific certainty of someone who had found the thing and understood what it was.
The Cup: intact Horcrux, a weapon for the final confrontation. A decision about what was more useful — destruction now or utility later. He had made the calculation and stood by it. The Cup would be destroyed when the time came.
He looked at the staff from Uganda leaning against the far wall, at the basilisk fang in its sealed case, at the Egyptian sand jar and the brass compass.
He thought about the specific accumulated weight of what was in this room — the things he had gathered since second year, the things that were records and the things that were weapons and the things that were simply objects that had been somewhere significant and had come to be here. The Elder Wand, which he was not placing here yet because there was still a war to get through with it.
He thought about what would be here when the war was over.
He thought: this room will outlast all of it. The locket will be here when the people who made the locket are gone. The diadem will be here when the last person who knew Rowena Ravenclaw is long beyond memory. The staff will be here. The sand from the eastern desert will be here.
Things in the right place, kept carefully, for whatever came after.
He closed the vault door.
He went back upstairs to where the household was doing its August evening work — his mother in the kitchen, the twins in the grounds, Harry and Ginny somewhere in the garden, Hermione in the library, Neville writing letters to Professor Sprout about the Boomslang skin timeline. The specific warm noise of people who had been in a place long enough to be at home in it.
He went to the kitchen.
His mother looked up when he came in. She had the quality she always had when he appeared in the kitchen in the evening — the specific warmth of someone whose default state included the people she loved being present and who noted their presence the way she noted the sun on a good day.
'You're in time to help with dinner,' she said.
'Good,' he said.
He washed his hands and picked up the knife and stood beside her at the board, and the kitchen did its ordinary August work around them, and the vault was sealed below and the Cup was contained and the war was what it was and would be what it would be.
But the kitchen was warm.
And his mother was there.
And that was, for tonight, entirely sufficient.