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We employ high resolution spectroscopy and spectropolarimetry to derive thephysical properties and magnetic characteristics of the multiple system HD164492C, located in the young open cluster M20. The spectrum reveals evidenceof 3 components: a broad-lined early B star (HD 164492C1), a narrow-lined earlyB star (HD 164492C2), and a late B star (HD 164492C3). Components C2 and C3exhibit significant ($>100$ km/s) bulk radial velocity variations with a periodof $12.5351(7)$ d that we attribute to eccentric binary motion around a commoncentre-of-mass. Component C1 exhibits no detectable radial velocity variations.Using constraints derived from modeling the orbit of the C2+C3 binary and fromsynthesis of the combined spectrum, we determine the approximate physicalcharacteristics of the components. We conclude that a coherent evolutionarysolution consistent with the published age of M20 implies a distance to thesystem of $0.9\pm 0.2$ kpc, corresponding to the smallest published values. Weconfirm the detection of a strong magnetic field in the combined spectrum. Thefield is clearly associated with the broad-lined C1 component of the system.Repeated measurement of the longitudinal magnetic field allows the derivationof the rotation period of the magnetic star, $P_{\rm rot}=1.36986(6)$ d. Wederive the star's magnetic geometry, finding $i=63\pm 6$ deg, $\beta=33\pm 6$deg and a dipole polar strength $B_{\rm d}=7.9^{+1.2}_{-1.0}$ kG. Strongemission - varying according to the magnetic period - is detected in theH$\alpha$ profile. This is consistent with the presence of a centrifugalmagnetosphere surrounding the rapidly rotating magnetic C1 component.

HD 164492C: a rapidly-rotating, H$\alpha$-bright, magnetic early B star associated with a 12.5d spectroscopic binary