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We report on FORS2 optical spectroscopy of the black hole X-ray binary V404Cygni, performed at the very beginning of its 2015 outburst decay, complementedby quasi-simultaneous $Swift$ X-ray and ultra-violet as well as REMnear-infrared observations. Its peculiar spectrum is dominated by a wealth ofemission signatures of HI, HeI, and higher ionisation species, in particularFeII. The spectral features are divided between broad red-shifted and narrowstationary varieties, the latter being emitted in the outer regions. Continuumand line variability at short time scale is high and we find Baldwineffect-like anti-correlations between the full-widths at half-maximum andequivalent widths of the broad lines with their local continua. The Balmerdecrement H{\alpha}/H{\beta} is also abnormally large at $4.61\pm0.62$. Weargue that these properties hint at the broad lines being optically thick andarising within a circumbinary component in which shocks between fasteroptically thick and slower optically thin regions may occur. We associate it toa nova-like nebula formed by the cooling remnant of strong accretion disc windsthat turned off when the mass-accretion rate dropped following the last majorflare. The FeII lines likely arise from the overlap region between this nebulaand the companion star winds, whereas we favour the shocks within the nebula asresponsible for the optical continuum via self-absorbed optically thinbremsstrahlung. The presence of a near-infrared excess also points towards thecontribution of a strongly variable compact jet or a dusty component.

The nova-like nebular optical spectrum of V404 Cygni at the beginning of the 2015 outburst decay