A Mammoth Clansman with the Fragment Disease had a potent concentration of an Empyrean Tusk blood in him.
Basically, among Mammoth Clansmen, he was the closest genetically to an Empyrean Tusk. That was why when two Mammoth Clansmen with the Fragment Disease copulate, they birth an Empyrean Tusk.
Of course, if the female didn't have enough foundation to nurture an Empyrean Tusk baby, she dies and the baby follows suit. Only those with a high cultivation at the Life Stage could birth a baby, and even then, there was a considerable risk of failure.
All Empyrean Tusks were birthed by Mammoth Clansmen.
The cure was very simple in essence. It was to mutate the genetic data of a Mammoth Clansman with the Fragment Disease to give that final push for them to become an Empyrean Tusk.