  We present SN2018kzr, the fastest declining supernova-like transient, second
only to the kilonova, AT2017gfo. SN2018kzr is characterized by a peak magnitude
of $M_r = -17.98$, peak bolometric luminosity of ${\sim} 1.4 \times 10^{43}$erg
s$^{\mathrm{-1}}$ and a rapid decline rate of $0.48 \pm 0.03$ mag
day$^{\textrm{-1}}$ in the $r$ band. The bolometric luminosity evolves too
quickly to be explained by pure $^{\mathrm{56}}$Ni heating, necessitating the
inclusion of an alternative powering source. Incorporating the spin-down of a
magnetized neutron star adequately describes the lightcurve and we estimate a
small ejecta mass of $M_\mathrm{ej} = 0.10 \pm 0.05$ $\textrm{M}_{\odot}$. Our
spectral modelling suggests the ejecta is composed of intermediate mass
elements including O, Si and Mg and trace amounts of Fe-peak elements, which
disfavours a binary neutron star merger. We discuss three explosion scenarios
for SN2018kzr, given the low ejecta mass, intermediate mass element composition
and the high likelihood of additional powering - core collapse of an
ultra-stripped progenitor, the accretion induced collapse of a white dwarf and
the merger of a white dwarf and neutron star. The requirement for an
alternative input energy source favours either the accretion induced collapse
with magnetar powering or a white dwarf - neutron star merger with energy from
disk wind shocks.
