  A black hole can launch a powerful relativistic jet after it tidally disrupts
a star. If this jet fortuitously aligns with our line of sight, the overall
brightness is Doppler boosted by several orders of magnitude. Consequently,
such on-axis relativistic tidal disruption events (TDEs) have the potential to
unveil cosmological (redshift $z>$1) quiescent black holes and are ideal test
beds to understand the radiative mechanisms operating in super-Eddington jets.
Here, we present multi-wavelength (X-ray, UV, optical, and radio) observations
of the optically discovered transient \target at $z=1.193$. Its unusual X-ray
properties, including a peak observed luminosity of $\gtrsim$10$^{48}$ erg
s$^{-1}$, systematic variability on timescales as short as 1000 seconds, and
overall duration lasting more than 30 days in the rest-frame are traits
associated with relativistic TDEs. The X-ray to radio spectral energy
distributions spanning 5-50 days after discovery can be explained as
synchrotron emission from a relativistic jet (radio), synchrotron self-Compton
(X-rays), and thermal emission similar to that seen in low-redshift TDEs
(UV/optical). Our modeling implies a beamed, highly relativistic jet akin to
blazars but requires extreme matter-domination, i.e, high ratio of
electron-to-magnetic field energy densities in the jet, and challenges our
theoretical understanding of jets.
