  One of the key open questions in Cosmology is the nature of the sources that
completed the cosmological hydrogen Reionization at z~5.2. High-z primeval
galaxies have been long considered the main drivers for Reionization, with a
minor role played by high-z AGN. However, in order to confirm this scenario, it
is fundamental to measure the photo-ionization rate produced by active SMBHs
close to the epoch of Reionization. Given the pivotal role played by
spectroscopically complete observations of high-z QSOs, in this paper we
present the first results of the RUBICON (Reionizing the Universe with BrIght
COsmological Nuclei) survey. It consists of a color selected sample of
bona-fide z~5 QSO candidates from the Hyper Suprime-Cam Subaru Strategic
Survey. Our QSO candidates have been validated both by photometric redshifts
based on SED fitting and by spectroscopic redshifts, confirming that they lie
at 4.5<z_spec<5.2. A relatively large space density of QSOs (Phi~1.4x10^-8
cMpc^-3) is thus confirmed at z~5 and M1450~-27, consistent with a pure density
evolution of the AGN luminosity function from z=4 to z=5, with a mild density
evolution rate of 0.25 dex. This indicates that AGN could play a non-negligible
role in the cosmic Reionization. The Rubicon of Reionization has been crossed.
