  Super-luminous supernovae (SLSNe) are rare events defined as being
significantly more luminous than normal terminal stellar explosions. The source
of the extra powering needed to achieve such luminosities is still unclear.
Discoveries in the local Universe (i.e. $z<0.1$) are scarce, but afford dense
multi-wavelength observations. Additional low-redshift objects are therefore
extremely valuable. We present early-time observations of the type I SLSN
ASASSN-18km/SN~2018bsz. These data are used to characterise the event and
compare to literature SLSNe and spectral models. Host galaxy properties are
also analysed. Optical and near-IR photometry and spectroscopy were analysed.
Early-time ATLAS photometry was used to constrain the rising light curve. We
identified a number of spectral features in optical-wavelength spectra and
tracked their time evolution. Finally, we used archival host galaxy photometry
together with HII region spectra to constrain the host environment.
ASASSN-18km/SN~2018bsz is found to be a type I SLSN in a galaxy at a redshift
of 0.0267 (111 Mpc), making it the lowest-redshift event discovered to date.
Strong CII lines are identified in the spectra. Spectral models produced by
exploding a Wolf-Rayet progenitor and injecting a magnetar power source are
shown to be qualitatively similar to ASASSN-18km/SN~2018bsz, contrary to most
SLSNe-I that display weak/non-existent CII lines. ASASSN-18km/SN~2018bsz
displays a long, slowly rising, red 'plateau' of $>$26 days, before a steeper,
faster rise to maximum. The host has an absolute magnitude of --19.8 mag ($r$),
a mass of M$_{*}$ = 1.5$^{+0.08}_{-0.33}$ $\times$10$^{9}$ M$_{\odot}$ , and a
star formation rate of = 0.50$^{+2.22}_{-0.19}$ M$_{\odot}$ yr$^{-1}$. A nearby
HII region has an oxygen abundance (O3N2) of 8.31$\pm$0.01 dex.
