  We present the first detailed analysis of the radio halo in the merging
galaxy cluster Abell 2256 using the LOFAR, uGMRT, and VLA. These observations
combined with archival X-ray data allowed us to study the halo emission with
unprecedented detail. The integrated radio emission from the entire halo is
characterized by an ultra-steep spectrum, which can be described by a power law
with $\alpha_{144 \rm MHz}^{1.5 \rm GHz}=-1.63\pm0.03$, and a radial steepening
in the outer regions. The halo is significantly underluminous according to the
scaling relations between radio power and mass at 1.4 GHz but not at 150 MHz;
ultra-steep spectrum halos are predicted to be statistically underluminous.
Despite the complex structure of this system, the radio halo morphology is
remarkably similar to that of the X-ray emission. The radio surface brightness
distribution across the halo is strongly correlated with the X-ray brightness
of the intracluster medium. The derived correlations show sublinear slopes and
there are distinct structures: the core is $\rm I_{R}\propto I_{X}^{1.51}$, the
outermost region $\rm I_{R}\propto I_{X}^{0.41}$, and we find radio
morphological connections with X-ray discontinuities. We also find a strong
anti-correlation between the radio spectral index and the X-ray surface
brightness, implying radial steepening. We suggests that the halo core is
either related to old plasma from previous AGN activity, being advected,
compressed and re-accelerated by mechanisms activated by the cold front or less
turbulent with strong magnetic field in the core. The change in the radio vs
X-ray correlation slopes in the outer regions of the halo could be due to a
radial decline of magnetic field, increase in the number density of seed
particles or increasing turbulence. Our findings suggest that that the emitting
volume is not homogenous according to turbulence re-acceleration models.
