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We present the discovery and properties of DESJ014132.4-542749.9(DES0141-54), a new powerful radio-loud active galactic nucleus (AGN) in theearly Universe (z=5.0). It was discovered by cross-matching the first datarelease of the Dark Energy Survey (DES DR1) with the Sidney University MolongloSurvey (SUMSS) radio catalog at 0.843 GHz. This object is the first radio-loudAGN at high redshift discovered in the DES. The radio properties of DES0141-54,namely its very large radio-loudness (R>10$^{4}$), the high radio luminosity(L$_{0.8 GHz}$=1.73$\times$10$^{28}$ W Hz$^{-1}$), and the flatness of theradio spectrum ($\alpha$=0.35) up to very high frequencies (120 GHz in thesource's rest frame), classify this object as a blazar, meaning, a radio-loudAGN observed along the relativistic jet axis. However, the X--ray luminosity ofDESJ0141-54 is much lower compared to those of the high redshift (z$\geq$4.5)blazars discovered so far. Moreover its X-ray-to-radio luminosity ratio(log($\frac{L_{[0.5-10]keV}}{L_{1.4GHz}}$)=9.96$\pm$0.30 Hz) is small also whencompared to lower redshift blazars: only 2\% of the low-z population has asimilar ratio. By modeling the spectral energy distribution we found that thispeculiar X--ray weakness and the powerful radio emission could be related to aparticularly high value of the magnetic field. Finally, the mass of the centralblack hole is relatively small (M$_{BH}$ = 3-8 $\times$10$^8$ M$_{\odot}$)compared to other confirmed blazars at similar redshift, making DES0141-54 theradio-loud AGN that host the smallest supermassive black hole ever discoveredat z$\geq$5.

An extremely X--ray weak blazar at z=5