Goliath Graveyard

Whenever a creature you control with power 5 or greater dies, you may add one mana of any colour to your mana pool. Until end of turn, this mana doesn't empty from your mana pool as steps and phases change.
From their bones, did form our mountains. From their blood, did form our rivers, our lakes. Their spirits, their souls birthed the seeds that would raise our forests. For their sacrifices, we shall honour them and chronicle their deeds, lest they be forgotten.