The strongest hero.

The simple answer is that even the Primordials can fall in love. And Ashu was no exception. He opened his soul to her, and she opened hers to him. And now, Sar-Elam has become a Dragon.

Haha. You might as well say they got married. I think there's no need to speculate any further in this discussion: the fact is that a mage became a Dragon, with Ashu's help. But according to official sources, Ashu enlightened Sar-Elam, and didn't engage in any kind of power transfer, at least I haven't seen anything about that, so, in fact, he acquired the power that surpassed Kha-Belekh himself, based solely on Ashu's knowledge.