Okay since you don't mind spoilers

LONG back when the white and black mages came together to start making Red magic, some of the Black Mages purposed summoning voidsent and drinking their blood, giving them great power. However those who did this would become mana-starved if they used too much in too short a time, causing them to because no more then hungry beasts preying on others to regain mana. Thus the practice was forbidden.

It should be noted it is NOT lore that Red Mages are vampires. You can if you want, but our powers have nothing to do with it.