Image by Carlos Rafael & Carlos Lopez.
At the end of the first book in the Barsoom series, A Princess of Mars, Mors Kajak and his father Tardos Mors took part in the celebration for the victory at Zodanga. Mors Kajak and his father were also present when John Carter and Dejah Thoris were married.
The celebration did not last long however, when the atmosphere factory was shut down, radical changes happened in the atmosphere and there was chaos. John Carter came to the atmosphere factory and investigated, he saved the planet from the atmosphere changes but it was generally believed that it had cost him his life. Only Dejah Thoris clung to the hope that John Carter was somehow still alive and back on Earth - as we know she was right.
Life went on for the royal House of Helium brightened by the childhood of John Carter and Dejah Thoris' son Carthoris. But then the young prince went missing. After about a year without word of any kind Tardos Mors and Mors Kajak took out a great fleet to search for their grandson/great grandson probing deep into the forbidden north polar region. But their fleet was brought down by the magnetic power of the Guardian of the North and were enslaved by the Yellow Men of Okar.
Fortunately John Carter's determined search for Dejah Thoris soon led him to Okar where he destroyed the Guardian and freed the prisoners including his father and grandfather in law.