Storm is a keyword ability.
Amy and Nate are playing and have between the two of them have cast 6 spells this turn. Amy then casts Empty the Warrens. Storm will then trigger and create 6 additional copies of Empty the Warrens. When the 7 spells have resolved Amy will have 14 goblin tokens.
- 702.39a. Storm is a triggered ability that functions on the stack. "Storm" means "When you cast this spell, put a copy of it onto the stack for each other spell that was cast before it this turn. If the spell has any targets, you may choose new targets for any of the copies."
- 702.39b. If a spell has multiple instances of storm, each triggers separately.