Eostra

Wait, the red lighting on her mask looks like a mouth, turning the fearsome Mage into a fucking Sesame Street character. 
Eostra, Queen of All Dark Things the Lurk in the Night, and former Eagle Owl mage, is the leader and most fearsome of all the Soul-Eaterz. She just about always wears an eagle owl mask.
Descriptions of five Soul-Eaters, including Eostra, were given in the original Wolf Brother. So you could argue that she originated from there, or not, or uhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh
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== In Soul Eater ==
When Torak disguised himself as a White Fox boy, he displaced some poor kid and got to meet Eostra for the first time. He had Nef, queen of deathpacito, to torment him more, though...
== In Ghost Hunter ==
As Thiazzi dropped the last fragment of the fire opal in his death plummet, Eostra's owls came to pick it up. Eostra then started an awful winter, brought out black dogs, made a bunch of Tokoroth with an alternate method, and summoned her fellow Soul-Eaters... the Walker said a little hello as well... nice of a madman todo ain't it? Eostra, carrying a scepter containing the fire opal, revealed that her plan was to absorb Torak's spirit walking power, thus allowing her to travel from being to being and become immortal. Torak, however, managed to shove his souls in her, allowing him to ridiculously survive a falling pile of rocks that killed Eostra, apparently, a great example of the book nerd's favorite term, plot armor, which perhaps mattered a little less at the end of the series, but still...