Disclaimer! I do not own Avatar the last airbender or the legend of Korra. All rights go to the creators.
This is kind of an AU. The simple plot is this. Avatar Roku had two children. One of his children became Ursa's mother. The other was a powerful firebender named Ash who had, in his rage, caused the volcano on Roku's island to erupt. Just before Roku died, he encased his son in molten lava and used every bit of his remaining strength to make sure his son was safe inside his shell. The molten lava was thrown back into the volcano and Roku's son was suspended in animation just like Aang.
Prologue
"Hurry, get out of here!" shouted Avatar Roku as the volcanic eruptions continued. Outside a very confused boy named Ash was staring in terror at the volcano he had somehow caused to erupt. Roku did his best to save his home but could not do it. After Sozin betrayed him, he lay on the ground dying. He suddenly noticed his son looking at him with tears in his eyes. Somehow, the volcanic heat and smoke did not affect him. Roku, in order to save his son, used up his remaining strength to encase his son in a shell made of molten lava. He then airbended the shell into the volcano after making sure the shell would endure for atleast 95 years. As the boy lost conscience, he heard his father say his final words. "Redeem me by watching over my descendants and making sure nothing happens to them or the next avatar".