The Zemo barony is a fictional barony that refers to the line of barons in the Zemo family of Marvel Comics. Two prominent members of the barony are Baron Heinrich Zemo and Baron Helmut Zemo who are the 12th and 13th barons in the line.
Harbin Zemo, the progenitor of the Zemo line was first seen, via flashback, in the Avengers/Thunderbolts limited series. The backstory of the eleven barons before Heinrich and Helmut was later touched upon in the limited series Thunderbolts Presents: Zemo - Born Better. So far it seems that every baron is succeeded by their son. According to Wendell Volker's notes Harbin had other children before Hademar who did not survive to adulthood; Herbert had two sons who died of illness prior to Helmuth.
Read more about Zemo Barony: Zemo Lineage, History