For the Mother River, a Level 7 Legendary Life merely has the qualifications to communicate with it, while a Level 8 Dominator is the true life form that can aid the development of the Mother River.
Herag can ascend to Level 7 through Bloodline Power, precisely because the Blood Rune within him has already been acknowledged by the Mother River and can directly communicate with it.
All of this is owed to the Thunder God Ancestral Bloodline, as long as the Ancestral Bloodline within Herag is powerful to a certain extent, he can directly ascend to Level 7.
This process won't face any obstacles because it comes from the inherited Power within the Bloodline.
This is also the reason why figures like the Thousand-Eyed Ancestor want to assist Herag.
