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 \ion Hi , \ion Hei , and higher ionisation species , in particular \ion 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 \pm 0.62 .
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 \ion 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 .