Maoshan Taoists are a staple of Chinese horror and vampire/kung fu movies, because a Maoshan Taoist is sort of a Chinese equivalent of a hoodoo doctor. They can curse people, fight vampires, do exorcisms and call up demons, so they're a natural choice for a spooky movie. The powers attributed to Maoshan Taoists in these movies are so over the top that one might be tempted to think they're totally fictional.
The Maoshan sect really does exist, and plenty of people in Taiwan or Hong Kong are pretty scared of them, even if they can't really control the legions of the undead. Maoshan talismans are commands to the spirits, signed with the seals and symbols of the Taoist celestial hierarchy, so people believe they can actually command spirits for good or for ill. There are even rumors of Triad crime bosses studying Maoshan Taoism in order to gain mental control over the prostitutes who work for them. Maoshan itself, though, is not an evil practice- it's just that some people believe the powers it gives you can be used for evil if you so choose. It's like the “dark side” of the Force!