Isn’t it crazy that Elgar’nan was the first spirit to successfully bind itself to matter? The will on this being to be able to so do.
And Elgar’nan, as the first spirit to take form, would have been so unbalanced with no one to reflect him. And he was. The earliest stories about him always describe him as raging and battling - pure will without reflection, power with no equal to temper it.
For Mythal, watching from the Fade as a spirit embodying justice or even benevolence, that must have been such a draw. To see this imbalance and to want to balance it. For her to choose to take form, leave the Fade and leave her companion spirit, meant that something in Elgar’nan drew her from the Fade. A different type of call to her purpose. And when she took form, it wasn’t to destroy him.
She “walked out of the sea of the earth’s tears and onto the land, placed her hand upon his brow- and at her touch, he grew calm.” Sounds like two new beings discovering each other, being seen in each other.
Mythal and Elgar'nan are two halves of a whole in material form. In him, she found purpose beyond abstraction as a spirit. She could be physically what she had always represented spiritually. For a spirit of Justice/Benevolence that would be a revelation - to exert that purpose in flesh. “We are the best of physical and spirit,” she would later say to Wisdom and I think she meant it. I think Mythal loved her form. And I think Elgar'nan loved his as well and together they reveled in each others forms for a time, reflecting each other, being seen. So I think Mythal left the Fade to join Elgar'nan out of fascination and purpose.
And can you imagine how fascinating Elgar'nan must have found Mythal as well? He, a being of pure dominance and power finding peace in her touch?
Together they formed a harmony in flesh - flame and water, sun and moon. Together they "brought back to life all the wondrous things that the sun had destroyed, and they grew and thrived" - building their pantheon together.
It makes their eventual fall as a the All-Father and All-Mother even more delicious.