The tangled roots of the great weirwood, and bound within it is the three-eyed raven, the last greenseer. The depiction in the show is much less fantastical than in the novel, where the last greenseer is a dessicated almost-corpse with roots growing through his body.
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Three-eyed raven, Three-eyed crow, Season 4, Last Greenseer, Greenseer, EP410 Profile, EP410