Gravestorm is a keyword ability.
- 702.68a Gravestorm is a triggered ability that functions on the stack. “Gravestorm” means “When you cast this spell, put a copy of it onto the stack for each permanent that was put into a graveyard from the battlefield this turn. If the spell has any targets, you may choose new targets for any of the copies.”
- 702.68b If a spell has multiple instances of gravestorm, each triggers separately.