You'll need the Fairy familiar for this (not the Faerie). The correct familiar will have a green outfit. Turn on the Fairy Card in the Relics menu. Kill some mooks until your Fairy is at level 5. Find a chair to sit in (I did this in the telescope room). There seems to be some RNG involved, but if luck is with you, the Fairy will sit on Alucard's shoulder and start singing.