The tiara Donut put on back on the first floor, the origin of everything the party is now bleeding for.
Carl traces the whole campaign back to a single act: Donut putting that goddamned tiara on her head way back on the very first floor. Everything since — the war, the borrowed spells, the permanent tattoos — stems from it.
Donut knows it, which is why her loud objections to Imani's barbed-wire tattoo read to Carl as guilt rather than vanity. "We were doing this for her, and she knew it." The blustering about the 1990s is how she expresses herself when she doesn't know how to react.
A small tiara worn atop Donut's head.
It is the item whose casual claiming set the entire campaign in motion.