MY EVIL CAMPAIGN

Thay has a strict and nuanced hierarchy of castes, with nobles at the highest levels of influence.  The vast majority of Red Wizards and higher level Thayan clerics come from noble backgrounds, and most of Thay's governors and enclave leaders do, as well.  Thayan nobles of Mulan blood are officially allowed to breed with other Mulan, outsiders, and some other races of interest (such as the Yuan-ti).  As Thayan enclaves have become fixtures in far-away cities, a number of Thayans have arranged sanctioned marriages with local nobles.  With each new marriage, and sometimes with individual births, nobles pledge their children to one house.

Thayan noble houses that do not have a main family business or pursuit tend to splinter, as different family members try to steer the house in too many directions.  Some of the current houses split off from each other, such as the vicious battles that split House Malaerach into House Malaergost and House Onolath.  While some noble houses are in direct competition due to their interests, other houses compete simply to be the most prestigious.  Family lands are highly prized, as as notable magical items.  Many commoners and slaves are attached to noble estates and feud with the servants of rival houses. 

Thay's oldest families are inevitably tied to the nobles of Mulhorhand, although some changed their names when Thay broke free from the god-kings.  A few point to their family ties in Unther, Murghom, and Semphar.  Every now and again, someone tries to trace their lineage all the way back to the Great Artificers of Imaskar, but such attempts are always hotly debated.