  We report on the discovery of AT2018lqh (ZTF18abfzgpl) -- a rapidly-evolving
extra-galactic transient in a star-forming host at 242 Mpc. The transient
g-band light curve's duration above half-maximum light is about 2.1 days, where
0.4/1.7 days are spent on the rise/decay, respectively. The estimated
bolometric light curve of this object peaked at about 7x10^42 erg/s -- roughly
seven times brighter than AT2017gfo. We show that this event can be explained
by an explosion with a fast (v~0.08 c) low-mass (~0.07 Msun) ejecta, composed
mostly of radioactive elements. For example, ejecta dominated by Ni-56 with a
time scale of t_0=1.6 days for the ejecta to become optically thin for
gamma-rays fits the data well. Such a scenario requires burning at densities
that are typically found in the envelopes of neutron stars or the cores of
white dwarfs. A combination of circumstellar material (CSM) interaction power
at early times and shock cooling at late times is consistent with the
photometric observations, but the observed spectrum of the event may pose some
challenges for this scenario. The observations are not consistent with a shock
breakout from a stellar envelope, while a model involving a low-mass ejecta
ramming into low-mass CSM cannot explain both the early- and late-time
observations.
