Donut's survival ability that fires when her health drops critically, usable only once per fight.
As Donut prepares to cast War Crime, Carl runs the numbers with her: the moment she casts, her health will drop by one percent, and he has a scroll ready to heal her. Her Cockroach ability shouldn't trigger at that threshold, but if it does, she probably won't be able to use it again for the rest of the fight.
That matters because of what comes next. If the Nest collapses before they can jump — a real possibility with the tower at eight percent health and an Anti-Escape Trap Net sealing them in — Donut would be left vulnerable exactly when the ability would otherwise save her, forcing her to rely on Hover and a potion instead.
Donut's once-per-fight survival trigger, at risk of being wasted on the War Crime cast.
Also appears in books you've already read — spoiler-safe to their endings: