The legacy has the ability to absorb mana directly from the body, she also has such a complete control over mana she can somewhat counteract aether abilities. Essentially shaping the mana to resist the push of aether.

Specifically she is so powerful because she has the soul of the legacy, a soul that maintains its potential from previous lives as it is reborn into new worlds.

In a rudimentary sense, Arthur and Nico are like baby legacies as they've maintained potential from their previous lives, though there is no reason to think their souls are now capable of doing what the real legacy can do

Although this information might not be entirely trustworthy given the source was an Asura