The Nightblood first appeared in the original release of EverQuest II. Not much is known about what they are or how they came to be other than a foot soldier in the "Void Invasion" of Norrath.
They speak in an infernal type language known as "Chaos Tongue".
Some powerful Nightblood have also been seen on Norrath. K'Dal the Deceiver in a special instance of Maiden's Gulch within Lavastorm. There is also N`Kzak the Unnatural in the Enchanted Lands along with General Bloodburn. Alegos The Betrayer resides in Rivervale along with Demeroth The Gnawer and Xelindros The Fearbringer. The Overfiend also frequents Rivervale as well. Lord Zaavan appears on a boatride quest. Archfiend Izzoroth takes up residence inside Nektropos Castle.
Some Nightblood found within the Enchanted Lands as well as in Rivervale seem to have an affinity with the faction called The Enchanted Reapers.
Even though these beasts seem to come from the Void, we do not know much more with the release of The Shadow Odyssey.