  We present multiband optical photometric and spectroscopic observations of an
unusual Type II supernova, SN 2018hfm, which exploded in the nearby (d = 34.67
Mpc) dwarf galaxy PGC 1297331 with a very low star-formation rate (0.0270 M
yr-1) and a subsolar metallicity environment(~ 0.5 Z). The V-band light curve
of SN 2018hfm reaches a peak with value of -18.69+/-0.64 mag, followed by a
fast decline(4.42+/-0.13 mag (100d)-1). After about 50 days, it is found to
experience a large flux drop (~3.0 mag in V), and then enters into an unusually
faint tail, which indicates a relatively small amount of 56Ni synthesized
during the explosion. From the bolometric light curve, SN 2018hfm is estimated
to have low ejecta mass (~1.3M) and low explosion energy(~10 50 erg) compared
with typical SNe II. The photospheric spectra of SN 2018hfm are similar to
those of other SNe II, with P Cygni profiles of the Balmer series and metal
lines, while at late phases the spectra are characterised by box-like profiles
of HU emission, suggesting significant interaction between the SN ejecta and
circumstellar matter. These box-like emission features are found to show
increasing asymmetry with time, with the red-side component becoming gradually
weaker, indicating that dust is continuously formed in the ejecta. Based on the
dust-estimation tool damocles, we find that the dust increases from ~10 -6M to
10 -4 - 10 -3 M between +66.7 d and +389.4 d after explosion.
