The War of Blood refers to the underground conflict between the Sabbat under Mordred and the Black Hand under the Antediluvians. They fought a cold war between their agents from 2203 to 2255 while both were simultaneously hunted by Earth Federation Crusader Teams.
Gehenna caused the rise of the antediluvians, but many of them were eventually destroyed by the Tech Infantry. Those that survived retreated to the Wraithlands, cementing their control over those vampires that survived the apocolypse. Mordred, a fourth-generation vampire, had gone into hiding rather than serve the antediluvians. Meanwhile, his enemies solidified their control over the vampire community. In 2196, the Black Hand makes a deal with the Earth Federation to allow ghouls to join the Tech Infantry. In exchange, they take ownership of a system (later called Enoch) on the rim of known space. Two years later, when the Wraithlands become uninhabitable thanks to the Bug Asteroid that wiped out Earth, the antediluvians moved out and formed the Kingdom of Enoch.
Battle of Wilke's Star
With the Black Hand leadership on the Rim, Mordred made his move. Quickly gathering many vampire dissidents who opposed the antediluvians, Mordred reformed the Sabbat, and began challenging vampire controlled areas within the Earth Federation. This led to more defections, which caused Ventrue, the King of Enoch to declare Mordred a wanted criminal, and offer a high price for his heart. Ventrue also encouraged his allies in the Fed to hunt for Mordred as well, including the recently rescued Bruce von Eisenstein. In 2201, von Eisenstein found him and attempted to capture the wily vampire, only to be destroyed in turn. Mordred fled to Wilke's Star and formed a new headquarters there.
Gathering his strength, he decided to turn the manhunt against Ventrue. Leaking information that Mordred was on Wilke's Star with a strong army, Ventrue himself came along with the 10th Tech Infantry Legion, manned solely by Black Hand ghouls. In 2203, this force proceeded to fight a vicious campaign on the planet and won, but the sacrifice of the Sabbat army was part of the ruse. The conflict allowed Mordred to reach Ventrue, defeated him, and then diablerized him, finally becoming an antediluvian himself.
Outraged at the loss of their king, the new Queen of Enoch, Arikel, declared war on the Sabbat by any means necessary. Of course, the use of a Tech Infantry legion by a foreign power finally broke the treaty between the Black Hand and the Federation, and ghouls were kicked out of the Fed military.
Vampires slaughtered each other throughout the Fed, often in public, terrorizing the citizenry. After several ships from the Kingdom of Enoch were caught sneaking through Federation space, the Grand Council realized that the War of Blood needed to be contained. In 2209, the Tech Infantry created the Crusader division, forcing both Black Hand and Sabbat adherents to be more discreet. So the hot war became a cold war, but no less vicious.
In 2225, Mordred decided to eliminate the Black Hand's interference in Fed affairs. He supported the The Resistance during the 2nd Civil War, which led to the TI's disasterous landing on Wilke's Star, but makes the name of then Brigadier-General Sarah Dunmeyer. That eliminated the Sabbat's active support in the human conflict, but the War of Blood continued on, led from that planet. In 2236, General Arthur Clarke is ordered to finally eradicate all vampires from the planet. Mordred is assumed to have died, but actually remained dormant, hidden under a pile of rubble.
Now believed to be dead, Mordred was able to operate the Sabbat more effectively, without interference from Crusader Teams. By 2241, his operatives were on the move again, including Fabian Cortez. He was able to infiltrate many of the Harrington daughter corporations and built their influence over the Fed. Through secretly manuevering many bureaucrats through proxies, he was able to convince Earth Fleet to finally eliminate the vampire threat from the Kingdom of Enoch. In 2255, they used a Dooms Day Device on the Enoch system, killing Arikel, and decapitating the Black Hand leadership. Without an antediluvian to serve, the organization fell apart, leaving Mordred supreme.
Behind the Scenes
The War of Blood was the key behind the "Ring of Fire" campaign as well as Fabian Cortez's plotline in Season 4.2, both run by Nathan Bax. Vampires have been rather marginalized in later campaigns.