  We report the discovery of a remarkable concentration of massive galaxies
with extended X-ray emission at $z_{spec} = 2.506$, which contains 11 massive
($M_{*} \gtrsim 10^{11} M_{\odot}$) galaxies in the central 80kpc region
(11.6$\sigma$ overdensity). We have spectroscopically confirmed 17 member
galaxies with 11 from CO and the remaining ones from $H\alpha$. The X-ray
luminosity, stellar mass content and velocity dispersion all point to a
collapsed, cluster-sized dark matter halo with mass $M_{200c} = 10^{13.9\pm0.2}
M_{\odot}$, making it the most distant X-ray-detected cluster known to date.
Unlike other clusters discovered so far, this structure is dominated by
star-forming galaxies (SFGs) in the core with only 2 out of the 11 massive
galaxies classified as quiescent. The star formation rate (SFR) in the 80kpc
core reaches $\sim$3400 $M_{\odot}$ yr$^{-1}$ with a gas depletion time of
$\sim 200$ Myr, suggesting that we caught this cluster in rapid build-up of a
dense core. The high SFR is driven by both a high abundance of SFGs and a
higher starburst fraction ($\sim25\%$, compared to 3\%-5\% in the field). The
presence of both a collapsed, cluster-sized halo and a predominant population
of massive SFGs suggests that this structure could represent an important
transition phase between protoclusters and mature clusters. It provides
evidence that the main phase of massive galaxy passivization will take place
after galaxies accrete onto the cluster, providing new insights into massive
cluster formation at early epochs. The large integrated stellar mass at such
high redshift challenges our understanding of massive cluster formation.
