  We present the peculiar case of PSZ2G091.83+26.11 at z=0.822. This cluster
hosts a Mpc-scale radio halo and an elongated radio source, whose location with
the respect to the intracluster medium (ICM) distribution and to the cluster
centre is not consistent with a simple merger scenario. We use VLA data at 1-4
GHz to investigate the spectral and polarisation properties of the diffuse
radio emission. We combine them with previously published data from LOFAR n the
120-168 MHz band, and from the uGMRT at 250-500 and 550-900 MHz. We also
complement the radio data with Chandra X-ray observations to compare the
thermal and non-thermal emission of the cluster. The elongated radio emission
is visible up to 3.0 GHz and has an integrated spectral index of
$-1.24\pm0.03$, with a steepening from $-0.89\pm0.03$ to $-1.39\pm0.03$. These
values correspond to Mach numbers $\mathcal{M}_{\rm radio,int}=3.0\pm0.19$ and
$\mathcal{M}_{\rm radio,inj}=2.48\pm0.15$. Chandra data reveals a surface
brightness discontinuity at the location of the radio source, with a
compression factor of $\mathcal{C}=2.22^{+0.39}_{-0.30}$ (i.e.
$\mathcal{M}_{\rm Xray}=1.93^{+0.42}_{-0.32}$). We also find that the source is
polarised at GHz frequencies. We estimate an intrinsic polarisation fraction of
$\sim0.2$, a Rotation Measure of $\sim50~{\rm rad~m^{-2}}$ (including the
Galactic contribution) and an external depolarisation of $\sim60~{\rm
rad~m^{-2}}$. The $B$-vectors are aligned with the major axis of the source,
suggesting magnetic field compression. Hence, we classify this source as a
radio relic. We also find a linear/super-linear correlation between the
non-thermal and thermal emission. We propose an off-axis merger and/or multiple
merger events to explain the position and orientation of the relic. Given the
properties of the radio relic, we speculate that PSZ2G091.83+26.11 is in a
fairly young merger state.

Astrophysics of Galaxies;Cosmology and Nongalactic Astrophysics