  The origin and evolution of magnetism in OB stars is far from being well
understood. With approximately 70 magnetic OB stars known, any new object with
unusual characteristics may turn out to be a key piece of the puzzle. We report
the detection of an exceptionally strong magnetic field in the He-strong B2IV
star CPD-62 2124. Spectropolarimetric FORS2 and HARPSpol observations were
analysed by two independent teams and procedures, concluding on a strong
longitudinal magnetic field of approximately 5.2 kG. The quantitative
characterisation of the stellar atmosphere yields an effective temperature of
23650$\pm$250 K, a surface gravity of 3.95$\pm$0.10 dex and a surface helium
fraction of 0.35$\pm$0.02 by number. The metal composition is in agreement with
the cosmic abundance standard, except for Mg, Si and S, which are slightly
non-solar. The strong and broad ($\sim$300 km/s) disc-like emission displayed
by the H$\alpha$ line suggests a centrifugal magnetosphere supported by the
strong magnetic field. Our results imply that CPD-62 2124 is an early B-type
star hosting one of the strongest magnetic fields discovered to date, and one
of the most evolved He-strong stars known, with a fractional main-sequence
lifetime of approximately 0.6.
