  In this paper, we present observations of two high-resolution transit
datasets obtained with ESPRESSO of the bloated sub-Saturn planet WASP-131~b. We
have simultaneous photometric observations with NGTS and EulerCam. In addition,
we utilised photometric lightcurves from {\tess}, WASP, EulerCam and TRAPPIST
of multiple transits to fit for the planetary parameters and update the
ephemeris. We spatially resolve the stellar surface of WASP-131 utilising the
Reloaded Rossiter McLaughlin technique to search for centre-to-limb convective
variations, stellar differential rotation, and to determine the star-planet
obliquity for the first time. We find WASP-131 is misaligned on a nearly
retrograde orbit with a projected obliquity of $\lambda = 162.4\substack{+1.3
\\ -1.2}^{\circ}$. In addition, we determined a stellar differential rotation
shear of $\alpha = 0.61 \pm 0.06$ and disentangled the stellar inclination
($i_* = 40.9\substack{+13.3 \\ -8.5}^{\circ}$) from the projected rotational
velocity, resulting in an equatorial velocity of $v_{\rm{eq}} =
7.7\substack{+1.5 \\ -1.3}$~km s$^{-1}$. In turn, we determined the true 3D
obliquity of $\psi = 123.7\substack{+12.8 \\ -8.0}^{\circ}$, meaning the planet
is on a perpendicular/polar orbit. Therefore, we explored possible mechanisms
for the planetary system's formation and evolution. Finally, we searched for
centre-to-limb convective variations where there was a null detection,
indicating that centre-to-limb convective variations are not prominent in this
star or are hidden within red noise.
