  The uncharted territory of the high-redshift ($z\gtrsim3$) Universe holds the
key to understand the evolution of quasars. In an attempt to identify the most
extreme members of the quasar population, i.e., blazars, we have carried out a
multi-wavelength study of a large sample of radio-loud quasars beyond $z=3$.
Our sample consists of 9 $\gamma$-ray detected blazars and 133 candidate
blazars selected based on the flatness of their soft X-ray spectra (0.3$-$10
keV photon index $\leq1.75$), including 15 with NuSTAR observations. The
application of the likelihood profile stacking technique reveals that the
high-redshift blazars are faint $\gamma$-ray emitters with steep spectra. The
high-redshift blazars host massive black holes ($\langle \log~M_{\rm
BH,~M{\odot}} \rangle>9$) and luminous accretion disks ($\langle L_{\rm disk}
\rangle>10^{46}$ erg s$^{-1}$). Their broadband spectral energy distributions
are found to be dominated by high-energy radiation indicating their jets to be
among the most luminous ones. Focusing on the sources exhibiting resolved X-ray
jets (as observed with the Chandra satellite), we find the bulk Lorentz factor
to be larger with respect to other $z>3$ blazars, indicating faster moving
jets. We conclude that the presented list of the high-redshift blazars may act
as a reservoir for follow-up observations, e.g., with NuSTAR, to understand the
evolution of relativistic jets at the dawn of the Universe.
