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The lowest possible mass of ONeMg white dwarfs (WDs) has not been clarifieddespite its importance in the formation and evolution of WDs. We tackle thisissue by studying the properties of V1405 Cas (Nova Cassiopeiae 2021), which isan outlier given a combination of its very slow light-curve evolution and therecently reported neon-nova identification. We report its rapid spectralevolution in the initial phase, covering 9.88, 23.77, 33.94, 53.53, 71.79, and81.90 hours after the discovery. The first spectrum is characterized by linesfrom highly-ionized species, most noticeably He II and N III. These lines arequickly replaced by lower-ionization lines, e.g., N II, Si II, and O I. Inaddition, Al II (6237 \r{A}) starts emerging as an emission line at the secondepoch. We perform emission-line strength diagnostics, showing that the densityand temperature quickly decrease toward later epochs. This behavior, togetherwith the decreasing velocity seen in H$\alpha$, H$\beta$, and He I, indicatesthat the initial nova dynamics is reasonably well described by an expandingfireball on top of an expanding photosphere. Interestingly, the strengths ofthe N III and Al II indicate large abundance enhancement, pointing to an ONeMgWD progenitor as is consistent with its neon-nova classification. Given itslow-mass nature inferred by the slow light-curve evolution and relativelynarrow emission lines, it provides a challenge to the stellar evolution theorythat predicts the lower limit of the ONeMg WD mass being $\sim$ 1.1 $M_\odot$.

Spectra of V1405 Cas at the very beginning indicate a low-mass ONeMg white dwarf progenitor