He had designed the rune work three nights before the schools arrived. He finalized it an hour after seeing the goblet.
The entrance hall at two in the morning had the specific kind of silence - not empty but present, the torchlight doing the work that torchlight did in stone corridors. The Goblet sat in the center of the age line circle, its cold blue-white flame the same at two in the morning as it had been at noon, indifferent to the hour.
He had thought carefully about the intervention and had decided against the obvious approach.
The obvious approach was to prevent a second Hogwarts name from entering at all — a constraint on the Goblet's selection mechanism that would simply discard any entry after the first. He had considered this through September and had found two problems with it. The first was detection: a modification to the Goblet's fundamental selection process would be visible to Dumbledore, who had built the age line and understood the Goblet's mechanics better than anyone currently living. The second was uncertainty — he did not know with confidence whose name would go in first. The Goblet had been in the entrance hall for days. Any Hogwarts student of age could have submitted a name already.
He needed a different condition.
He had arrived at it in the second week of October, working through the runic architecture of the Goblet's selection logic with the patience of someone who had been studying Ancient Runes for two years and had been thinking about this specific problem for three months.
The Goblet selected based on two criteria: school affiliation and worthiness, where worthiness was the Goblet's own assessment of the submitter's fitness for the competition. The age line was a precondition on submission, not on selection — it prevented underage students from entering, but the Goblet's selection logic operated on the pool of valid entries afterward.
What he needed was not a constraint on the first entry or the most recent entry. He needed a constraint on the selection criteria themselves.
The modification he designed added a single condition to the Hogwarts selection logic: among Hogwarts entries and any of a fourth school, the Goblet would select the candidate it assessed as most worthy who was also not yet of age.
This was a narrower target than he had initially wanted, but it had the advantage of being defensible on its own terms — the Goblet was still selecting from valid entries, still assessing worthiness, still making its own judgment. He was not overriding the selection. He was adding a preference for a specific subset of the eligible pool.
The problem was the same uncertainty: he did not know whether Harry would enter. In the original timeline Harry had not entered voluntarily — his name had been submitted by Crouch Junior. Crouch Junior was here, wearing Moody's face, and would presumably do the same thing. But he could not be certain of this. He could not be certain of the timing. He could not be certain that the submission, when it happened, would produce an entry the Goblet assessed as worthy in the specific way the Goblet assessed things.
He had sat with this uncertainty for two weeks.
Then he had made the decision that he usually made when a perfect solution was not available: he would do the thing that was most likely to produce the outcome he needed, accept that it was not guaranteed, and prepare for the contingency in which it failed.
He cast without a wand, in the wandless style he had been developing since Uagadou. The rune sequence settled into the base of the Goblet with the specific quality of work that had been done correctly — not visible on the surface, present in the structure, the kind of modification that would not show up in a standard diagnostic because it was not a modification to the Goblet's external behavior but to a preference weighting deep in its selection logic.
He stood back and checked it once.
Then he looked at the Goblet and thought: if Crouch Junior submits Harry's name, this ensures it is selected. If Crouch Junior does not submit Harry's name, this does nothing. If Harry submits his own name voluntarily, this ensures it is selected. If Harry does not submit his own name, and Crouch Junior does not submit it either — then the Goblet selects whoever it selects, and the Tournament proceeds without Harry, and a great many things change in ways he cannot calculate.
He had decided this was an acceptable risk. The alternative was attempting to force Harry's selection in a way that did not depend on Crouch Junior's actions, which would require a far more invasive modification and would be visible.
He walked back to Gryffindor Tower and went to sleep.
On Halloween, if Dumbledore read the third slip of parchment and said Harry Potter, Ron would feel the specific quality of a contingency that had resolved correctly — not relief, because he had been managing the anxiety about this since September and had decided that managing it was more useful than feeling it, but the quiet acknowledgement of a thing going as intended.
He filed it and turned his attention to the next problem.