He spent the evening researching the knife.
This knife, like the sword of Gryffindor, and like any other made of goblin steel, had the property of imbibing that which strengthened it. The sword had taken basilisk venom in the Chamber and become, thereby, one of the very few objects capable of destroying a Horcrux. The principle was the same for any Goblin-made blade: the steel absorbed what it was used against, retained it, was changed by it.
He had a sealed vial of basilisk venom in the fifth compartment of his trunk.
The logic was straightforward. He worked through it twice to make sure he was not missing anything, then spent an hour on the specific technique for introducing venom to a blade without destroying the blade or himself in the process. The venom would need to be applied carefully — not to the full blade, which would make it permanently dangerous to handle, but to the first inch of the point, enough to deliver the necessary penetration to a Horcrux without making the knife impossible to carry.
He went to the potions room in the Burrow's basement in his memory, which was not the same as going there in practice but was adequate for working through the problem theoretically. The application would need a specific temperature, a specific implement — glass, not metal — and a very steady hand.
He had a steady hand.
He made a note in his notebook and went to bed.
He was not in bed for long.
The problem kept him for another two hours — not the application technique, which he had worked out adequately, but the broader question the knife had opened. He had the means to destroy Horcruxes now, in a form more practical than carrying a basilisk fang. The fang was in his trunk; it was adequate as a last resort. The knife was better — smaller, portable, concealable, the kind of thing you could carry without announcing what you were carrying.
He sat at the desk in the study configuration of the Room and made the list he had been carrying in his head since the Chamber of Secrets.
The diary: destroyed, June of this year. The locket: destroyed this week. The diadem: destroyed this week. That was three.
The ring: he knew where it was. Little Hangleton, the Gaunt shack, a cursed object set in a ring that had been Marvolo Gaunt's and before that Peverell's. He could not go to Little Hangleton. He was thirteen years old, he did not have a way to travel there without raising suspicion , and even if he had, the ring was heavily cursed in a way that had cost Dumbledore his hand in the original timeline. He filed this one under not yet.
The cup: in Bellatrix Lestrange's vault at Gringotts. Also not yet, and the not-yet here was structural rather than temporary — there was no version of his current self that could retrieve something from a Gringotts high-security vault. He put a mark against this one and left it.
Nagini: not a Horcrux yet. Would become one. He did not know precisely when the snake would be made into a Horcrux — it was not information that had been clear in the books — but the timeline was not imminent. He put this one in a separate category: watch, not act.
Harry: he was careful with this one. He had been careful with it since he had first understood it, because it was the most uncomfortable item on the list and the one he had the least clear thinking about. Harry was an accidental Horcrux — a fragment of Voldemort's soul had attached itself to him the night the Killing Curse failed, and it would need to be addressed before Voldemort could be killed, and the canon solution had been Harry dying and surviving through the Deathly Hallows and the specific mechanics of a Killing Curse cast by someone who did not have the right to cast it.
He was not going to let Harry die. This was the fixed point. Everything else was negotiable; this was not.
Which meant he needed an alternative, and he did not have one yet. He had been thinking about it since June and had arrived at several theoretical frameworks and no practical solution. The problem was that the fragment of soul in Harry had attached itself to the living structure of Harry's magical self; removing it required either killing the host or finding a method of separation that had never, as far as he could determine from his reading, been documented.
He had more reading to do.
He wrote this at the bottom of the list — Harry: alternative needed — and looked at the list for a long moment.
Seven confirmed Horcruxes, one destroyed last June, two more would be destroyed by the end of the month. Four remaining that he knew of, one that was not yet made, two he would have to prepare for as one had defeated even Dumbledore and the other was stealing from one of the safest buildings in wizarding Britain and the last one that was not a conventional Horcrux and could not be destroyed by conventional means.
And finally there was Voldemort himself, who was currently less than a person and more than nothing, growing toward the graveyard and the cauldron and the return that was approximately eighteen months away.
Eighteen months was not very long.