I finally watched the full "Hashira Meeting" animation by illuxxxtrandy , and I am absolutely floored. The way the tension was captured? Chef’s kiss.

Illuxxxtrandy’s gaze lifted, and for the first time the men and women at the table noticed threads of fatigue in the wanderer’s face—small sacrifices worn like embroidery. "Because this demon learned to speak the names of children who survived your hunts. It calls them by pet names. It tells their mothers where to find lost keepsakes. The villagers start to trust it. They stop telling the truth to one another. A fracture of that kind spreads faster than any plague."

When the creature emerged, it wore the shape of a woman who had once been beloved in the valley—soft eyes, hands folded—yet the light in those eyes was too even, too rehearsed. It spoke the names it had learned. The villagers murmured, some in comfort, some in doubt. Illuxxxtrandy stepped forward without a blade and began to tell a story of a night many years ago, a story only half-true, threaded with small, private details misaligned on purpose. The demon replied in those borrowed cadences; but as Illuxxxtrandy skewed the facts just enough, phrases began to unravel. The borrowed memories stopped matching. The creature’s voice flickered.

On the eve of the final battle, all surviving Hashira gather at the Ubuyashiki mansion. Muzan attacks, leading to the explosion of Kagaya’s compound and the Hashira being scattered into the Infinity Castle.