Diana of the Ephesians was the goddess of fertility and was depicted as many-breasted, hence Aziraphale’s comment. She is also mentioned in Acts 19:28, where Ephesian metal-smiths shouted her defence, “Great is Diana of the Ephesians” against St Paul’s preaching of Christianity.
One of her four hands holds a sword: this may be the Shiv Aziraphale refers to.
Brian’s quotation is almost but not quite there. It should be “a plague on both your houses”.
Mercutio utters this line in Shakespeare’s Romeo and Juliet after he has been mortally wounded.