  The non-linear and non-local effects in atomic Rydberg media under
electromagnetically induced transparency (EIT) make it a versatile platform for
fundamental studies and applications in quantum information. In this paper, we
study the dynamics of a Rydberg-EIT system in an ensemble that allows for more
than one Rydberg excitation in the propagation direction. The density of
two-level atoms is such that transient superradiant effects occur. We
experimentally observe a cross-over between coherent collective emission
(`flash') of two-level atoms to a Rydberg dressed regime ('dressed flash')
under EIT condition. The complex dynamics are characterised using both
intensity and time correlation measurements. We show that while steady-state
EIT gives a second order correlation $g^{(2)}=0.79\pm 0.04$, the
Rydberg-dressed flash exhibits anti-bunching down to $0.2\pm0.04$.
