  We employ high resolution spectroscopy and spectropolarimetry to derive the
physical properties and magnetic characteristics of the multiple system HD
164492C, located in the young open cluster M20. The spectrum reveals evidence
of 3 components: a broad-lined early B star (HD 164492C1), a narrow-lined early
B star (HD 164492C2), and a late B star (HD 164492C3). Components C2 and C3
exhibit significant ($>100$ km/s) bulk radial velocity variations with a period
of $12.5351(7)$ d that we attribute to eccentric binary motion around a common
centre-of-mass. Component C1 exhibits no detectable radial velocity variations.
Using constraints derived from modeling the orbit of the C2+C3 binary and from
synthesis of the combined spectrum, we determine the approximate physical
characteristics of the components. We conclude that a coherent evolutionary
solution consistent with the published age of M20 implies a distance to the
system of $0.9\pm 0.2$ kpc, corresponding to the smallest published values. We
confirm the detection of a strong magnetic field in the combined spectrum. The
field is clearly associated with the broad-lined C1 component of the system.
Repeated measurement of the longitudinal magnetic field allows the derivation
of the rotation period of the magnetic star, $P_{\rm rot}=1.36986(6)$ d. We
derive 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. Strong
emission - varying according to the magnetic period - is detected in the
H$\alpha$ profile. This is consistent with the presence of a centrifugal
magnetosphere surrounding the rapidly rotating magnetic C1 component.
