
So, basically, the heron will give her immortality, but because she's human, her heart can't accept that much mana in such a small body. So the mana will be given to her heart in small quantities, and over time, the heart will slowly transform into that of a dragon. Unfortunately, because she is human, her body can only withstand so much, and after five years, she will die as she is supposed to.
I think that's what it means, from my understanding.

From my understanding, her body will become immortal, except for her heart? Because, as the dragon had said, the mana will be too much for a human to take for a long period of time, so it'll eventually literally break her heart. 5 years was a time indicator that the dragon gave them. So she will be immortal until her heart eventually collapses under the pressure of the mana.
can somebody explain to me. She will live for an additional 5 years, but her body will be immortal due to the fusion? so is it like she'll lose her self after 5 years and become monster like? I don't get it. Or is it that the body will become immortal-like during the course of 5 years?