Every mortal has to come with a sponsor mage, and once migrated is considered "Officially Dead" on Earth. To maintain the sense of familial ties and closeness everyone within a mage's entourage/cabal/family/group, took on the sponsor mage's last name.
Some "families" are closer than others. Some have very distinct goals and ideals, others don't. Some are held in high regard thanks to their well-regarded sponsor/leader/head of house/etc, while others are dealing with a disgraced sponsor.
Below are the list of "family" surnames that are available, click on any surname to read about that family.