I would say something like
anima draconis Ravennatis, usque ab die cum Marius Magnus eum devicerit, domum nostram est tuita.
lit. the spirit of the Ravennian dragon (I thought it would be nice to use the adjective, seeing as it exists), has protected our family (I used household here, as it is perhaps the wider sense that is meant: property as well as people?) ever since the day when Marius the great completely defeated it.