WordNet (r) 3.0 (2006):

Svengali
    n 1: someone (usually maleficent) who tries to persuade or force
         another person to do his bidding
    2: the musician in a novel by George du Maurier who controls
       Trilby's singing hypnotically

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "Svengali":
   Mesmer, Rasputin, VIP, access, bad influence, big wheel, court,
   eminence grise, five-percenter, friend at court, good influence,
   gray eminence, heavyweight, hidden hand, hypnotist, hypnotizer,
   influence, influence peddler, influencer, ingroup, key, kingmaker,
   lobby, lobbyist, lords of creation, man of influence, manipulator,
   mesmerizer, open sesame, powers that be, pressure group,
   sinister influence, special interests, special-interest group,
   the Establishment, very important person, wheeler-dealer,
   wire-puller