The loot box from killing Empress D'Nadia that produced both Run, Little Günter, Run and War Crime.
Carl recalls that Donut's War Crime spell came out of the Legendary 1914 box, the box they received after killing Empress D'Nadia — the same box from which he got his Spellbook of Run, Little Günter, Run spell. He finds the sheer difference in capacity between the two spells astonishing: one a battlefield curiosity, the other something the system labels an Atrocity.
The loot box that armed Donut with the spell at the center of Carl's plan.