How bad is it? Not so terrible that there wasn't a Witchcraft XI... but it's still one of the lowest rated films of all time.
Should you see it? Maybe. Have a lot of drinks, watch it with others who like making fun of films.
Some film series continue just through sheer inertia and this is from one of those. The worst - by far - of the Witchcraft films,this has bisexual vampires from England releasing a Satanist from the police, causing them to be hunted by both the local cops (LAPD) and Interpol. The sound effects are not in synch - often off by more than a second! The police station looks like somebody's apartment, most of the cops don't have uniforms, one cop is miscast way too young and another is so fat his clothes don't fit (which may have had nothing to do with the film). There is a ton of nudity; that's why this was made and some of it is not terrible.