  We report the discovery of ROXA J081009.9+384757.0 = SDSS J081009.9+384757.0,
a z=3.95 blazar with a highly unusual Spectral Energy Distribution (SED). This
object was first noticed as a probable high $f_x/f_r$, high-luminosity blazar
within the error region of a $\approx 10^{-12}$ erg/s cm$^2$ ROSAT source
which, however, also included a much brighter late-type star. We describe the
results of a recent Swift observation that establishes beyond doubt that the
correct counterpart of the X-ray source is the flat spectrum radio quasar. With
a luminosity well in excess of $10^{47}$ erg/s, ROXA J081009.9+384757.0 is
therefore one of the most luminous blazars known. We consider various
possibilities for the nature of the electromagnetic emission from this source.
In particular, we show that the SED is consistent with that of a blazar with
synchrotron power peaking in the hard X-ray band. If this is indeed the case,
the combination of high-luminosity and synchrotron peak in the hard-X-ray band
contradicts the claimed anti-correlation between luminosity and position of the
synchrotron peak usually referred to as the "blazar sequence". An alternative
possibility is that the X-rays are not due to synchrotron emission, in this
case the very peculiar SED of ROXA J081009.9+384757.0 would make it the first
example of a new class of radio loud AGN.
