We report the discovery of the unusually bright long-duration gamma-ray burst (GRB), GRB221009A, as observed by the Neil Gehrels Swift Observatory (Swift), Monitor of All-sky X-ray Image, and Neutron Star Interior Composition Explorer Mission. This energetic GRB was located relatively nearby (z=0.151), allowing for sustained observations of the afterglow. The large x-ray luminosity and low Galactic latitude (b=4°.3) make GRB221009A a powerful probe of dust in the Milky Way. Using echo tomography, we map the line-of-sight dust distribution and find evidence for significant column densities at large distances(≥10kpc). We present analysis of the light curves and spectra at X-ray and UV–optical wavelengths, and find that the X-ray afterglow of GRB221009A is more than an order of magnitude brighter at T0+4.5 ks than that from any previous GRB observed by Swift. In its restframe, GRB221009A is at the high end of the afterglow luminosity distribution, but not uniquely so. In a simulation of randomly generated bursts, only 1 in 10⁴ long GRBs were as energetic as GRB221009A; such a large E_γ,iso implies a narrow jet structure, but the afterglow lightcurve is inconsistent with simple top-hat jet models. Using the sample of Swift GRBs with redshifts, we estimate that GRBs as energetic and nearby as GRB221009A occur at a rate of ≤1 per 1000yr^-1 making this a truly remarkable opportunity unlikely to be repeated in our lifetime.