  $\tau$ Sco, a well-studied magnetic B-type star in the Upper Sco association,
has a number of surprising characteristics. It rotates very slowly and shows
nitrogen excess. Its surface magnetic field is much more complex than a purely
dipolar configuration which is unusual for a magnetic massive star. We employ
the CMFGEN radiative transfer code to determine the fundamental parameters and
surface CNO and helium abundances. Then, we employ MESA and GENEC stellar
evolution models accounting for the effects of surface magnetic fields. To
reconcile $\tau$ Sco's properties with single-star models, an increase is
necessary in the efficiency of rotational mixing by a factor of 3 to 10 and in
the efficiency of magnetic braking by a factor of 10. The spin down could be
explained by assuming a magnetic field decay scenario. However, the
simultaneous chemical enrichment challenges the single-star scenario. Previous
works indeed suggested a stellar merger origin for $\tau$ Sco. However, the
merger scenario also faces similar challenges as our magnetic single-star
models to explain $\tau$ Sco's simultaneous slow rotation and nitrogen excess.
In conclusion, the single-star channel seems less likely and versatile to
explain these discrepancies, while the merger scenario and other potential
binary-evolution channels still require further assessment as to whether they
may self-consistently explain the observables of $\tau$ Sco.
