  In this work, we present photometric and spectroscopic data of the
low-luminosity Type IIP supernova (SN) 2018hwm. The object shows a faint
($M_r=-15$ mag) and very long ($\sim$130 days) plateau, followed by a 2.7 mag
drop in the $r$-band to the radioactive tail. The first spectrum shows a blue
continuum with narrow Balmer lines, while during the plateau the spectra show
numerous metal lines, all with strong and narrow P-Cygni profiles. The
expansion velocities are low, in the 1000-1400 km s$^{-1}$ range. The nebular
spectrum, dominated by H$\alpha$ in emission, reveals weak emission from [O I]
and [Ca II] doublets. The absolute light curve and spectra at different phases
are similar to those of low-luminosity SNe IIP. We estimate that 0.0085
$M_{\odot}$ of $^{56}$Ni mass were ejected, through hydrodynamical simulations.
The best fit of the model to the observed data is found for an extremely low
explosion energy of 0.075 foe, a progenitor radius of 845 $R_{\odot}$ and a
final progenitor mass of 9-10 $M_{\odot}$. Finally, we performed a modeling of
the nebular spectrum, to establish the amount of oxygen and calcium ejected. We
found a low M($^{16}$O)$\approx 0.02$ $M_{\odot}$, but a high M($^{40}$Ca) of
0.3 $M_{\odot}$. The inferred low explosion energy, the low ejected $^{56}$Ni
mass and the progenitor parameters, along with peculiar features observed in
the nebular spectrum, are consistent with both an electron-capture SN explosion
of a super-asymptotic giant branch star and with a low-energy, Ni-poor iron
core-collapse SN from a 10-12 $M_{\odot}$ red supergiant.
