A malodorous wizard hat looted from a skunk that taught Donut the atrocity spell Rolling Battle Formation.
Pressed by Mordecai on what she had just cast, Donut explains she got Rolling Battle Formation from that skunk's hat — the stinky wizard hat — and that it is an atrocity spell. The hat is the sole source she names for the spell.
Particularly with the tower amplifying it, the hat's spell proves far more powerful than Donut anticipated, sweeping tens of thousands of witnesses and two gods into a single rolling sphere and earning her a public Atrocity notification.
A wizard's hat taken from a skunk, notably and memorably stinky.
The loot item that put an atrocity-grade mass spell in Donut's hands.