On legendary each team has a necromancer who can raise the dead from either of the two graveyards on the map. When 8 minutes are left you can move your necromancer into the pentagram at either of the graveyards, and you will summon undead units to help you in the game. Either necromancer can use either graveyard, but once one graveyard has supplied corpses for the fight, it is empty and can not be used by the other team. This means if you play your cards right you can get both sets of reinforcements well making sure your opponent gets none.