Gnawa as a term has meaning within meaning. It references a Moroccan ethnic minority descended from slaves taken out of sub-Saharan African empires over 500 years ago. Yet it is also a minority within itself: the fewer than 300 people who practice the Lila Derdeba ritual ceremony of spirit possession. Don’t get creeped out, though; these are musicians/healers whose entrancing sound borrows from Berber tradition, African shamanism and Arabic Islam, and it has caught non-Gnawa ears all over. The Altaf Gnawa Group has four members but sounds like 40. Their music is led by the low-pitched, rhythmic guembri (also called gimbri, sentir or hajhouj), a long-necked, three-stringed lute three or four feet in length. The melodies are intentionally hypnotic (for induction of healing trances), and driven by call-and-response singing, hand-clapping, and the large metal qraqebs (similar to castanets). Think Casablanca blues and spiritual music. Spine-tingling stuff, and that’s the point.