In the anime, instead of giving a corpse their blood, the Royals would instead insert a small pinkishish flame into their body to resurrect them. This was done in order to cut down on the blood and gore in the anime. Like blood warriors, the resurrected corpses must have their flames replenished once every few days or they'll die permanently.
In the beginning, all members of the Royalty can only resurrect corpse using their blood. However, once a Royal matures to a point to be able to utilize phoenix fire, he or she may choose to plant a portion of that sacred flame inside their blood warriors, turning them into flame warriors who are truly immortal. Other than immortality, the flame warriors can also utilize the phoenix flames in battle. The flame warrior may in time choose to pass on his or her own Flame of Life to someone else, this turns the recipient into a flame warrior but permanently kills the giver.