Radiohead has unveiled a new song! Hours after a couple of teasing Instagram posts floated onto the Web — one of a bluebird, the other of a medievally clad woman being tied to a tree by men in True Detective Season 1 masks — the band debuted “Burn the Witch,” a very promising herald of the band’s ninth album, due whenever it decides to drop it. It’s a great portent, a very balanced blending of Thom Yorke’s soulful mature vocal style and Jonny Greenwod’s increasingly focused, classically informed musical approach.”Burn the Witch” has been a rumor (and a bootleg) among Radiohead fans for years. This version is grounded in strings strongly evoking Greenwood’s tributes to the Polish composer Krzysztof Penderecki, but with the pop emotionalism of his film-score work. Radiohead’s other members add heft to the steady crescendo that builds to some trademark Yorke falsetto rhapsodizing.He’s crooning our doom, of course: The song’s lyrics scorn the kind of communal complicity that Yorke sees as leading to environmental disaster. As the video shows in cute Claymation-style harkening back to the cult-classic film The Wicker Man, the round robin of blame captured in lines like “stay in the shadows, cheer the gallows” doesn’t solve anything, only adding to the “low flying panic attack” that “Burn the Witch” expertly evokes. Radiohead has been making beautiful music about the end of the world for a long time; this offering suggests that the new album will join in that mission as one of the band’s most stirring calls to act.