  We report on FORS2 optical spectroscopy of the black hole X-ray binary V404
Cygni, performed at the very beginning of its 2015 outburst decay, complemented
by quasi-simultaneous $Swift$ X-ray and ultra-violet as well as REM
near-infrared observations. Its peculiar spectrum is dominated by a wealth of
emission signatures of HI, HeI, and higher ionisation species, in particular
FeII. The spectral features are divided between broad red-shifted and narrow
stationary varieties, the latter being emitted in the outer regions. Continuum
and line variability at short time scale is high and we find Baldwin
effect-like anti-correlations between the full-widths at half-maximum and
equivalent widths of the broad lines with their local continua. The Balmer
decrement H{\alpha}/H{\beta} is also abnormally large at $4.61\pm0.62$. We
argue that these properties hint at the broad lines being optically thick and
arising within a circumbinary component in which shocks between faster
optically thick and slower optically thin regions may occur. We associate it to
a nova-like nebula formed by the cooling remnant of strong accretion disc winds
that turned off when the mass-accretion rate dropped following the last major
flare. The FeII lines likely arise from the overlap region between this nebula
and the companion star winds, whereas we favour the shocks within the nebula as
responsible for the optical continuum via self-absorbed optically thin
bremsstrahlung. The presence of a near-infrared excess also points towards the
contribution of a strongly variable compact jet or a dusty component.
