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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). Thisobject was first noticed as a probable high $f_x/f_r$, high-luminosity blazarwithin the error region of a $\approx 10^{-12}$ erg/s cm$^2$ ROSAT sourcewhich, however, also included a much brighter late-type star. We describe theresults of a recent Swift observation that establishes beyond doubt that thecorrect counterpart of the X-ray source is the flat spectrum radio quasar. Witha luminosity well in excess of $10^{47}$ erg/s, ROXA J081009.9+384757.0 istherefore one of the most luminous blazars known. We consider variouspossibilities for the nature of the electromagnetic emission from this source.In particular, we show that the SED is consistent with that of a blazar withsynchrotron 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 bandcontradicts the claimed anti-correlation between luminosity and position of thesynchrotron peak usually referred to as the "blazar sequence". An alternativepossibility is that the X-rays are not due to synchrotron emission, in thiscase the very peculiar SED of ROXA J081009.9+384757.0 would make it the firstexample of a new class of radio loud AGN.Comment: 7 pages, 5 figures, A&A in pres

ROXA J081009.9+384757.0: a $\mathsf{10^{47}}$ erg s$\mathsf{^{-1}}$ blazar with hard X-ray synchrotron peak or a new type of radio loud AGN?