Hook: The players will be in a major city, hunting down a master vampire and his minions when they discover someone else seems to be helping them out. In some cases, the characters will arrive only minutes after a pitched battle to discover the remains of scores of vampires. Whoever the mystery hunter or hunters are, they are strong and able to hold their own against the bloodsuckers.
Line: At some point in a fight with secondary vampires, the players will start to lose big time. A strange figure will streak into combat to aid the players. When the carnage is over, the players will find themselves facing their savior: another vampire.
His name is Steven St. George, a vampire unlike any others. He will attempt to talk to the players and will avoid combat. If he is attacked, he will retreat, to try to settle things later, after proving himself. He explains that he is the result of some demonic experiment. He claims that he was kidnapped by some sort of doctor who was trying to use vampire blood to create super-soldiers, a formula he would sell to the military after perfecting. Steven was kept in a cell until his turn as a guinea pig but saw several of the failures, horrid zombie-like creations which were destroyed.
After his ordeal, which was excruciating and nightmarish, he was locked into an airtight plastic case to await results. While helpless in the case, he saw Rajin, the mater vampire the doctor had been using for samples. Rajin broke free from a similar case and killed the doctor. In the fight, Stevens case was cracked, by the time he broke free, Rajin was gone. Steven has tracked him to this city and plans to destroy the other vampire (it is Rajin the characters have been hunting as well). He will help the players in any way he can.
Sinker: Steven is a puppet. Rajin is a master vampire, but not the source of Steven's change. It is a complex plot by another master vampire to eliminate some competition. The second vampire (at the orchestration of it's controlling intelligence) made an illusion to appear as Rajin. Steven was changed into a secondary vampire and set free to hunt down the enemy. Right now Steven has his own free will (better to draw in allies and make his attacks unexpected), but at any time the intelligence behind the vampire can move in to take over.
The intelligence has been subliminally guiding Steven in the right direction. It will attempt to have Steven befriend the party to provide help in eliminating Rajin. When Rajin is taken care of, the intelligence will crush what remains of Stevens lingering personality and humanity, then turn on the party to try kill them off by surprise. There is no hope for Steven, he is doomed. After the intelligence takes over, he will no longer exist.
If you are feeling particularly evil, after this adventure is played out you might have the party meet up with a free-willed vampire or wampyr.