  ASASSN-18am/SN 2018gk is a newly discovered member of the rare group of
luminous, hydrogen-rich supernovae (SNe) with a peak absolute magnitude of $M_V
\approx -20$ mag that is in between normal core-collapse SNe and superluminous
SNe. These SNe show no prominent spectroscopic signatures of ejecta interacting
with circumstellar material (CSM), and their powering mechanism is debated.
ASASSN-18am declines extremely rapidly for a Type II SN, with a
photospheric-phase decline rate of $\sim6.0~\rm mag~(100 d)^{-1}$. Owing to the
weakening of HI and the appearance of HeI in its later phases, ASASSN-18am is
spectroscopically a Type IIb SN with a partially stripped envelope. However,
its photometric and spectroscopic evolution show significant differences from
typical SNe IIb. Using a radiative diffusion model, we find that the light
curve requires a high synthesised $\rm ^{56}Ni$ mass $M_{\rm Ni}
\sim0.4~M_\odot$ and ejecta with high kinetic energy $E_{\rm kin} = (7-10)
\times10^{51} $ erg. Introducing a magnetar central engine still requires
$M_{\rm Ni} \sim0.3~M_\odot$ and $E_{\rm kin}= 3\times10^{51} $ erg. The high
$\rm ^{56}Ni$ mass is consistent with strong iron-group nebular lines in its
spectra, which are also similar to several SNe Ic-BL with high $\rm ^{56}Ni$
yields. The earliest spectrum shows "flash ionisation" features, from which we
estimate a mass-loss rate of $ \dot{M}\approx 2\times10^{-4}~\rm
M_\odot~yr^{-1} $. This wind density is too low to power the luminous light
curve by ejecta-CSM interaction. We measure expansion velocities as high as $
17,000 $ km/s for $H_\alpha$, which is remarkably high compared to other SNe
II. We estimate an oxygen core mass of $1.8-3.4$ $M_\odot$ using the [OI]
luminosity measured from a nebular-phase spectrum, implying a progenitor with a
zero-age main sequence mass of $19-26$ $M_\odot$.
