Thirteen thieves who number themselves after the legs and body of a spider, and who replace the dead rather than mourn them.
Filed by its count, not its title
Reduced as a phrase, “The Phantom Troupe” would sit on The Ledger — but that would be reducing wording this reference chose. The set is filed under 4 · The Structure instead, because 13 is what its author chose, and a chosen number outranks an imposed one.
A troupe of thirteen thieves, each tattooed with a spider and a number. Membership is by number: when a member dies the number is filled, and the group's own rule is that the spider survives the loss of any leg.
Making the number a slot rather than a person is a genuinely unusual move, and the series builds a moral argument on it. A set that replaces its members is a set that has decided its members are interchangeable, and the story's antagonists say so out loud. Every other numbered set here treats its number as a description; this one treats it as a policy.
It also makes the set a source of tension rather than a checklist. The reader is never sure which thirteen are current, which is the opposite of what a numbered roster usually provides.
Thirteen is eight legs, four more, and a body — the count does not map neatly onto the animal, and the mismatch is left unresolved. Numbered sets built on a real-world referent tend to break like this. The author keeps the motif and lets the arithmetic go, which is the honest trade.