  The structure of the accretion flow onto supermassive black holes (SMBH) is
not well understood. Standard disc models match to zeroth order in predicting
substantial energy dissipation within optically-thick material producing a
characteristic strong blue/UV continuum. However they fail at reproducing more
detailed comparisons to the observed spectral shapes along with their observed
variability. Based on stellar mass black holes within our galaxy, accretion
discs should undergo a transition into an X-ray hot, radiatively inefficient
flow, below a (mass scaled) luminosity of $\sim 0.02\,L_{\rm{Edd}}$. While this
has been seen in limited samples of nearby low-luminosity active galactic
nuclei (AGN) and a few rare changing-look AGN, it is not at all clear whether
this transition is present in the wider AGN population across cosmic time. A
key issue is the difficulty in disentangling a change in spectral state from
increased dust obscuration and/or host galaxy contamination, effectively
drowning out the AGN emission. Here we use the new eROSITA eFEDS Survey to
identify unobscured AGN from their X-ray emission, matched to excellent optical
imaging from Subaru's Hyper Suprime-Cam; allowing the subtraction of the host
galaxy contamination. The resulting, uncontaminated, AGN spectra reveal a
smooth transition from a strongly disc dominated state in bright AGN, to the
collapse of the disc into an inefficient X-ray plasma in the low luminosity
AGN, with the transition occurring at $\sim 0.02\,L_{\rm{Edd}}$; revealing
fundamental aspects of accretion physics in AGN.
