The World Spark

As The World Spark enters the battlefield, you may pay X life, where X is equal to the number of lands in your graveyard. If you don't, sacrifice The World Spark.
{t}: Add X mana in any combination of colors, where X is equal to the number of lands in all graveyards.
{t}, Sacrifice The World Spark: Return all lands from all graveyards to their owner's battlefield.