  V838 Mon has undergone one of the most mysterious stellar outbursts on
record. The spectrum at maximum closely resembled a cool AGB star, evolving
toward cooler temperatures with time, never reaching optically thin conditions
or a nebular stage. The latest spectral type recorded is M8-9. The amplitude
peaked at DeltaV=9 mag, with the outburst evolution being characterized by a
fast rise, three maxima over four months, and a fast decay (possibly driven by
dust condensation in the ejecta). BaII, LiI and s-element lines were prominent
in the outburst spectra. Strong and wide (500 km/sec) P-Cyg profiles affected
low ionization species, while Balmer lines emerged to modest emission only
during the central phase of the outburst. A light-echo discovered expanding
around the object constrains its distance to 790+/-30 pc, providing M_V=+4.45
in quiescence and M_V=-4.35 at optical maximum (dependent on the still
uncertain E(B-V)=0.5 reddening). The visible progenitor resembled a somewhat
under-luminous F0 main sequence star, that did not show detectable variability
over the last half century. V838 Mon together with M31-RedVar and V4332 Sgr
seems to define a new class of astronomical objects: stars that erupt into cool
supergiants (SECS). They do not develop optically thin or nebular phases, and
deep P-Cyg profiles denounce large mass loss at least in the early outburst
phases. Their visible progenitors are located close to the Main Sequence, away
from the post-AGB region. After the outburst, the remnants still closely
resemble the precursors (same brightness, same spectral type). Many more
similar objects could be buried among poorly studied novae that have been
classified as Miras on the base of a single spectrum at maximum brightness.
