  Most violent and energetic processes in our universe, including mergers of
compact objects, explosions of massive stars and extreme accretion events,
produce copious amounts of X-rays. X-ray follow-up is an efficient tool for
identifying transients because (1) X-rays can quickly localize transients with
large error circles, and (2) X-rays reveal the nature of transients that may
not have unique signatures at other wavelengths. In this white paper, we
identify key science questions about several extragalactic multi-messenger and
multi-wavelength transients, and demonstrate how X-ray follow-up helps answer
these questions
