In the 1940's, Hannibal King was a private investigator making a decent living. He was hired to investigate an industrial espionage case. While investigating the factory in question, Hannibal King was killed when Deacon Frost killed everyone there.
Hannibal the Vampire
King awoke on a slab in the morgue three days later, finding himself one of the undead. Horrified with what he had become, King swore not to consummate his curse by feeding on another human. Hannibal lived on blood from blood banks, corpses, and animals. King chose not to use his vampire powers, as he felt that he lost a part of himself with each use. King remained a private investigator, working only at night, and specializing in occult cases, until he and Blade finally found Deacon Frost and destroyed him together.
Hannibal King was created by Marv Wolfman and Gene Colan and first appeared in Tomb of Dracula #25 (1974).
The Montesi Formula
When Doctor Strange used the Darkhold to cast the Montesi Formula, all the vampires on Earth were destroyed, and it became impossible for more people to be turned to vampires. Because Hannibal had never fed on a human, he was not destroyed by the Montesi Formula, although he did require a full blood transfusion to survive. After the transfusion, King was back to being a human. He remained a normal human until the Montesi Formula began weakening decades later, at which point he regained his vampire strengths and weaknesses; although he only craved human blood, he didn't need it to live.
With the Nightstalkers
With the weakening of the Montesi Formula, Doctor Strange organized Blade, Frank Drake, and Hannibal King into the Nightstalkers to battle Dracula and any other vampires that may return. They would later be teamed up again to fight Lilith. After the battle against her they formed Borderline Investigations to protect to world from the occult. During this time King would not see eye to eye for a while. He would also team up with Morbius and him and Drake fought Stone Cold and Vic Slaughter. At one point King was the first to be killed by Blade when he was Switchblade during the Midnight Massacre but was later brought back to life by Prof. Louise Hastings. During one mission, he and the NIghtstalkers fought the Lilin Short Circuit. During this battle, Short Circuit would trap King and Blade in an alternate universe where KIng was ruler of the vampires. He would also take part in the Siege of Darkness on Ghost Rider's team. He would then spy on Morbius to make sure Bloodthirst was not still in his body but also later team up with him to find Radar who is a member of the Blood. When the Nightstalkers battled Varnae, Hannibal's mind was taken over in an attempt to kill the team. Rather than letting himself be used as a weapon against his friends, King impaled himself on a pole. King was believed to be killed by the impalement and the explosion Drake caused to kill Varnae.
Although it was believed that King had been killed, he later reappeared in New Orleans and helped Blade defeat a resurrected Deacon Frost. King was then hired by Donna Garth to find her father, Simon Garth. King found Simon in New York, where he, with the help of Spider-Man, rescued Garth from Lilith.
What's King doing now?
Most recently, King started a private investigating firm in San Francisco. He battled Blade, after he refused his father's offer to save the souls of all vampires (which would effectively make them invincible as well). The story arch ended with Blade killing Hannibal only to end up reviving him again. It officially ended with Blade giving Hannibal a cure to his vampirism.
King has the standard powers associated with vampires. He has superhuman strength, speed, agility, stamina, senses, healing, and slowed/halted aging. Hannibal can change his body into mist, allowing him to squeeze through extremely tight spaces and fly, as well as into a large wolf. Hannibal can also control the minds of rats, and he uses them to gather information while he rests during the day. He can also use them as weapons to swarm on an enemy and eat them alive.
Hannibal King made his big-screen debut in the movie Blade: Trinity, played by Ryan Reynolds. Hannibal King was still a member of the Nightstalkers, although the team itself had changed greatly. The movie version of Hannibal had not been a PI, had been turned by a vampire named Danica Talos, and had been cured by a serum created for Blade in the first movie. Though this version had also been a vampire, he did not retain any superhuman abilities, and instead battled vampires using firearms and hand-to-hand combat. He carried a pair of pistols with digital video cameras built into the frame so that he could keep track of vampires that he battled.