Historical Rules · 5 October 2018
Rule 702.68a
Official source text from the 5 October 2018 Rules release
Gravestorm is a triggered ability that functions on the stack. “Gravestorm” means “When you cast this spell, copy it 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.”