Lucas' aether core was a Divine class, meeting the prerequisite of having, at least, a single perfect affinity with any Aether.
According to the tale, once the Earth Core was about to transfigure to become a Heaven Core, it would only recognize a single Aether with an existence who had a perfect affinity with it. It resulted in their body to be reconstructed and rebirthing to an element.
It was similar to a spirit.
That way, their abilities were not restricted anymore.
If the Archduke could summon light, he could only conjure one but could never become one.
Once he became a light, even physical objects could not touch him.
Simply put, he was invincible.
It was the basic understanding of a Heaven Realm existence.
There had been no tales, or even from Lucas Madrigal, the Devil who had attained even the higher stages of the Heaven Realm, of someone possessing two perfect affinities with different aether.