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The True Tongue is the language of the children of the forest, in which they sing the songs of the earth. The name the children call themselves in the True Tongue is those who sing the song of earth. No human man can speak the True Tongue, but ravens can.
A Dance with Dragons
Bran Stark hears Coldhands murmur a blessing in a strange tongue when the ranger slits the throat of his fallen elk. Within the cave of the three-eyed crow, Bran hears Leaf and other children of the forest sing in the True Tongue.