Unearth is a keyword ability.
Unearth is an ability which allows you to actually return a card from your graveyard to the battlefield for one turn. Not only does it get haste, but the unearth cost usually costs less than the casting cost of the card itself. The down side is that the card is removed from the game at the end of your turn, and you can only unearth during your main phases (as a sorcery). Some cards, like Grixis Slavedriver and Corpse Connoisseur, actually combo with unearth because they give benefits when they come into play or leave play.
- 702.82a Unearth is an activated ability that functions while the card with unearth is in a graveyard. "Unearth [cost]" means "[Cost]: Return this card from your graveyard to the battlefield. It gains haste. Exile it at the beginning of the next end step. If it would leave the battlefield, exile it instead of putting it anywhere else. Activate this ability only any time you could cast a sorcery."