  Accreting X-ray pulsars (XRPs) undergo luminous X-ray outbursts during which
the luminosity-dependent spectral and timing features of the neutron star's
emission can be analyzed in detail, thus shedding light on the accretion regime
at work. We took advantage of a monitoring campaign performed with NuSTAR,
Swift/XRT, AstroSat and NICER, to follow the Be/X-ray Binary 2S 1553-542 along
one of its rare outbursts and trace its spectral and timing evolution. We
report the discovery of a luminosity-dependent cyclotron line energy for the
first time in this source. The pulse profiles and pulsed fraction also show
variability along the outburst, consistently with the interpretation that the
source transitions from the sub-critical to the super-critical accretion
regime, separated by a critical luminosity of L$_{crit}\approx4\times10^{37}$
erg/s.
