  We present a detailed compilation and analysis of the X-ray phase space of
low- to intermediate-redshift ($ 0\le z \le 1$) transients that consolidates
observed light curves (and theory where necessary) for a large variety of
classes of transient/variable phenomena in the 0.3--10 keV energy band. We
include gamma-ray burst afterglows, supernovae, supernova shock breakouts and
shocks interacting with the environment, tidal disruption events and active
galactic nuclei, fast blue optical transients, cataclysmic variables, magnetar
flares/outbursts and fast radio bursts, cool stellar flares, X-ray binary
outbursts, and ultraluminous X-ray sources. Our overarching goal is to offer a
comprehensive resource for the examination of these ephemeral events, extending
the X-ray duration-luminosity phase space (DLPS) to show luminosity evolution.
We use existing observations (both targeted and serendipitous) to characterize
the behavior of various transient/variable populations. Contextualizing
transient signals in the larger DLPS serves two primary purposes: to identify
areas of interest (i.e., regions in the parameter space where one would expect
detections, but in which observations have historically been lacking) and to
provide initial qualitative guidance in classifying newly discovered transient
signals. We find that while the most luminous (largely extragalactic) and least
luminous (largely Galactic) part of the phase space is well-populated at $t >
0.1$ days, intermediate luminosity phenomena (L$_x = 10^{34} - 10^{42}$ erg
s$^{-1}$) represent a gap in the phase space. We thus identify L$_x = 10^{34} -
10^{42}$ erg s$^{-1}$ and $t = 10^{-4} - 0.1$ days as a key discovery phase
space in transient X-ray astronomy.
