Katia's unspent god-killing bolt, deliberately withheld from Prince Maestro and saved for a better target.
Princess Formidable gave the Bolt of Ophiotaurus to Katia Grim to use against Prince Maestro. Katia regretted not firing it during the trainyard fight, but Carl told her it would have been a waste: Maestro had been right that they couldn't have hurt him, and the gift was more likely a message from Formidable to her brother than a genuine assassination order.
The bolt therefore remains in Katia's possession, unspent, with Carl promising they would get to use it eventually.
A single legendary bolt, still unfired.
Chekhov's weapon — deliberately conserved for a future, worthier target.