Bill Scott said:
It happened in a several issue story arc in Dr Strange, and a couple Avengers, back in the late 80s if I recall correctly. It could have been the early 90s. I don't remember the details but in the end of the story line, I recall seeing panels of various marvel vampires, including Lilith, disolving into smoke as the spell was cast.
According to the OHOTMU book of the dead, it took place in Doctor Strange #62, so I think it was during the 80s, yeah.
Basically, Strange used the Montesi formula in the Darkhold to wipe out all vampires living on earth at the time, good or evil. As Hand of Vecna pointed out, King wasn't affected as he'd never tasted human blood (or at least living human blood). Pseudo-vampire Morbius wasn't affected either.
However, during a period in the 90s, following on the success of the Ghost Rider series, Marvel started several "Horror" titles, among them a series called Darkhold and one featuring the "triumvirate" of vampire hunters- Frank Drake, Blade, and Hannibal King. Sometime around here (actually slightly before, as otherwise having vampire hunters probably wouldn't make sense), the Montesi formula was undone by someone- I think it might have been connected to Doc Strange's brother, who had been in suspended animation as HoV again pointed out. He had revived and begun to act as a vigilante superhero (under the moniker Baron Blood- the second to take on that title), and I think that somehow undid the effects of the spell.
In any event, the old Vamp crew was back, and vampires could again exist on Marvel earth. Lilith came back (though she's evil now), Dracula came back, and King reverted to a vampire.
At some point during their series, King was merged with Frank Drake... again, not clear on how that happened or how it turned out, but the last I saw of King he was no longer merged with Drake, but still a vampire, and still hunting down other vamps.