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Observations of individual massive stars, super-luminous supernovae,gamma-ray bursts, and gravitational-wave events involving spectacularblack-hole mergers, indicate that the low-metallicity Universe is fundamentallydifferent from our own Galaxy. Many transient phenomena will remain enigmaticuntil we achieve a firm understanding of the physics and evolution of massivestars at low metallicity (Z). The Hubble Space Telescope has devoted 500 orbitsto observe 250 massive stars at low Z in the ultraviolet (UV) with the COS andSTIS spectrographs under the ULLYSES program. The complementary ``X-ShootingULLYSES'' (XShootU) project provides enhanced legacy value with high-qualityoptical and near-infrared spectra obtained with the wide-wavelength coverageX-shooter spectrograph at ESO's Very Large Telescope.  We present an overview of the XShootU project, showing that combining ULLYSESUV and XShootU optical spectra is critical for the uniform determination ofstellar parameters such as effective temperature, surface gravity, luminosity,and abundances, as well as wind properties such as mass-loss rates in functionof Z. As uncertainties in stellar and wind parameters percolate into manyadjacent areas of Astrophysics, the data and modelling of the XShootU projectis expected to be a game-changer for our physical understanding of massivestars at low Z.  To be able to confidently interpret James Webb Space Telescope (JWST) spectraof the first stellar generations, the individual spectra of low Z stars need tobe understood, which is exactly where XShootU can deliver.

X-Shooting ULLYSES: massive stars at low metallicity. I. Project Description