'Gravestorm' was introduced in the Time Spiral block and it currently appears on only one card: Bitter Ordeal. It may appear again in some future set.

702.66. Gravestorm

• 702.66a. 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."
• 702.66b. If a spell has multiple instances of gravestorm, each triggers separately.

Translation:

• 702.66a - When you cast the spell, you can copy it for each permanent that went to the graveyard this turn.
• 702.66b - Gravestorm stacks.