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We present the first extensive radio to gamma-ray observations of afast-rising blue optical transient (FBOT), AT2018cow, over its first ~100 days.AT2018cow rose over a few days to a peak luminosity $L_{pk}\sim4\times 10^{44}$erg/s exceeding those of superluminous supernovae (SNe), before declining as$\propto t^{-2}$. Initial spectra at $\lesssim 15$ days were mostly featurelessand indicated large expansion velocities v~0.1c and temperatures reaching 30000K. Later spectra revealed a persistent optically-thick photosphere and theemergence of H and He emission features with v~sim 4000 km/s with no evidencefor ejecta cooling. Our broad-band monitoring revealed a hard X-ray spectralcomponent at $E\ge 10$ keV, in addition to luminous and highly variable softX-rays, with properties unprecedented among astronomical transients. An abruptchange in the X-ray decay rate and variability appears to accompany the changein optical spectral properties. AT2018cow showed bright radio emissionconsistent with the interaction of a blastwave with $v_{sh}$~0.1c with a denseenvironment ($\dot M\sim10^{-3}-10^{-4}\,M_{\odot}yr^{-1}$ for $v_w=1000$km\s). While these properties exclude Ni-powered transients, ourmulti-wavelength analysis instead indicates that AT2018cow harbored a "centralengine", either a compact object (magnetar or black hole) or an embeddedinternal shock produced by interaction with a compact, dense circumstellarmedium. The engine released $\sim10^{50}-10^{51.5}$ erg over $\sim10^3-10^5$ sand resides within low-mass fast-moving material with equatorial-polar densityasymmetry ($M_{ej,fast}\lesssim0.3\,\rm{M_{\odot}}$). Successful SNe fromlow-mass H-rich stars (like electron-capture SNe) or failed explosions fromblue supergiants satisfy these constraints. Intermediate-mass black-holes aredisfavored by the large environmental density probed by the radio observations.

An embedded X-ray source shines through the aspherical AT2018cow: revealing the inner workings of the most luminous fast-evolving optical transients