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Izaak is a little bit mh... disturbing, when he smiles, right?XD

In addition to healing properties, vampires blood has the power to create ties. After accepting blood from a vampire three times, one becomes under the supernatural influence or mastery of the supplying vampire.
Link for more info: Blood Bond

Regarding the "Masquerade" this is a sort of campaign which aims to convince the mortal that vampies do not exist.
Link: Masquerade