Donut's spell that sweeps combatants into one enormous rolling sphere, credited by Carl with saving half the army.
As Donut agonizes over the day's losses, Carl tells her flatly that the deaths belong to the dead alien driving Eris, and that by casting Battle Formation she saved everyone — without it, half the army would already be dead. The spell's product is the Rolling Battle Formation Ball, with crawlers, The Reavers, The Dream soldiers, and gods all packed inside together.
Its cost is psychological as much as tactical: Donut can hear everyone trapped inside screaming, says nobody has mentioned how awful it is, and hopes they don't remember their time in there and aren't all angry with her. The ball's contents being mixed and immobile is exactly what makes the War Crime strike possible.
The crowd-control spell that concentrates the battlefield into a single breakable sphere.