  The lowest possible mass of ONeMg white dwarfs (WDs) has not been clarified
despite its importance in the formation and evolution of WDs. We tackle this
issue by studying the properties of V1405 Cas (Nova Cassiopeiae 2021), which is
an outlier given a combination of its very slow light-curve evolution and the
recently reported neon-nova identification. We report its rapid spectral
evolution in the initial phase, covering 9.88, 23.77, 33.94, 53.53, 71.79, and
81.90 hours after the discovery. The first spectrum is characterized by lines
from highly-ionized species, most noticeably He II and N III. These lines are
quickly replaced by lower-ionization lines, e.g., N II, Si II, and O I. In
addition, Al II (6237 \r{A}) starts emerging as an emission line at the second
epoch. We perform emission-line strength diagnostics, showing that the density
and temperature quickly decrease toward later epochs. This behavior, together
with the decreasing velocity seen in H$\alpha$, H$\beta$, and He I, indicates
that the initial nova dynamics is reasonably well described by an expanding
fireball on top of an expanding photosphere. Interestingly, the strengths of
the N III and Al II indicate large abundance enhancement, pointing to an ONeMg
WD progenitor as is consistent with its neon-nova classification. Given its
low-mass nature inferred by the slow light-curve evolution and relatively
narrow emission lines, it provides a challenge to the stellar evolution theory
that predicts the lower limit of the ONeMg WD mass being $\sim$ 1.1 $M_\odot$.
