Gravestorm

Gravestorm is a triggered keyword ability. It's similar to storm, except that instead of counting the number of spells casted that turn, it counts the number of permanents put into a graveyard from the battlefield that turn.

Description
Gravestorm was introduced in Future Sight and has only been printed on one card so far, the timeshifted Bitter Ordeal. According to Mark Rosewater, it is unlikely to be reused, since it is having the same problems as storm

During design, the mechanic was called Deathstorm.