  We propose a novel theoretical model to describe a physical identity of the
soft X-ray excess, ubiquitously detected in many Seyfert galaxies, by
considering a steady-state, axisymmetric plasma accretion within the innermost
stable circular orbit (ISCO) around a black hole (BH) accretion disk. We extend
our earlier theoretical investigations on general relativistic
magnetohydrodynamic (GRMHD) accretion which has implied that the accreting
plasma can develop into a standing shock for suitable physical conditions
causing the downstream flow to be sufficiently hot due to shock compression. We
numerically calculate to examine, for sets of fiducial plasma parameters, a
physical nature of fast MHD shocks under strong gravity for different BH spins.
We show that thermal seed photons from the standard accretion disk can be
effectively Compton up-scattered by the energized sub-relativistic electrons in
the hot downstream plasma to produce the soft excess feature in X-rays. As a
case study, we construct a three-parameter Comptonization model of inclination
angle $\theta_{\rm obs}$, disk photon temperature $kT_{\rm in}$ and downstream
electron energy $kT_e$ to calculate the predicted spectra in comparison with a
60 ks {\it XMM-Newton}/EPIC-pn spectrum of a typical radio-quiet Seyfert 1 AGN,
Ark~120. Our $\chi^2$-analyses demonstrate that the model is plausible in
successfully describing data for both non-spinning and spinning BHs with the
derived range of $61.3~{\rm keV} \lesssim kT_e \lesssim 144.3~{\rm keV}$,
$21.6~{\rm eV} \lesssim kT_{\rm in} \lesssim 34.0~{\rm eV}$ and $17.5\degr
\lesssim \theta_{\rm obs} \lesssim 42.6\degr$ indicating a compact Comptonizing
region of $3-4$ gravitational radii that resembles the putative X-ray coronae.
