There are two sets of Novels in Trinity Blood, Reborn on the Mars (R.O.M.) and Rage Against The Moons (R.A.M.). The events in Rage Against the Moons take place before those in Reborn on The Mars. The books were written by Yoshida Sunao (and after his death, continued by Kentaro Yasui) with illustrations done by THORES Shibamoto. There are also 5 manga volumes currently available; the anime is produced by GONZO and there are 24 episodes in total. Similar in feel to a Hellsing or Trigun. The background is in the distant future, a post-apocalyptic world after the destruction brought about by Armageddon. The war between humans (Terran) and vampires (Methuselah) continues, as a struggle between two factions: the Vatican and the New Human Empire. Yet amidst the bloodshed and violence, many desire a peaceful co-existence between the two species. However, standing between them and this goal is the Rosenkreuz Orden, a group of vampire extremists who manipulate the two sides and pit them against each other. To combat them, the Vatican special operations group "AX", led by Cardinal Caterina, must use everything they have - even a Crusnik (a vampire who preys on the blood of other vampires) to protect the peace effort. 
 
(Source: Tokyopop)

The background is in the distant future after the destruction brought about by Armageddon. The war between the vampires and the humans continue to persist. In order to protect the humans from the vampires, the Vatican has to rely on other allies to counter the situation. The protagonist,  Abel Nightroad, is a traveling priest from the Vatican and a crusnik, a vampire that drinks the blood of vampires. He is a member of the "Ax", a special operations group led by Cardinal Catherina Sforza. He encounters a young girl called Esther, who decides to go with him to Rome and train at the Vatican. Soon after he meets her, the order of Rozencreuz, led by Abel's twin, Cain, tries to continue the war so they can rule the world. It's up to Abel and the AX to try and stop them.

In a dark and distant future, Armageddon has given rise to the fabled Second Moon - and a perpetual war between the vampires and the humans!
 
Esther is a nun in the city of Istavan. When she crosses paths with Abel Nightroad, a priest sent from the Vatican to combat the local order of vampires, the two form a holy alliance to battle the most evil of threats. In this gorgeous, gothic-action series - part of the super-popular Trinity Blood franchise - the very survival of the human race is at stake!
 
(Source: Viz Media)

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