2025MNRAS.537.2662E

We present a sample of over 100 highly X-ray luminous galaxy clusters ($L_{\rm X, 0.1-2.4\, keV} \sim 10^{45}$ erg s$^{-1}$) at $z\sim 0.5$-0.9, discovered by the extended Massive Cluster Survey (eMACS) in ROSAT All-Sky Survey (RASS) data. Follow-up observations of a subset at higher resolution and greater depth with the Chandra X-ray Observatory are used to map the gaseous intracluster medium, while strong-gravitational-lensing features identified in Hubble Space Telescope imaging allow us to constrain the total mass distribution. We present evidence of the exceptional gravitational-lensing power of these massive systems, search for substructure along the line of sight by mapping the radial velocities of cluster members obtained through extensive ground-based spectroscopy, and identify dramatic cases of galaxy evolution in high-density cluster environments. The available observations of the eMACS sample presented here provide a wealth of insights into the properties of very massive clusters ($M_{200}\sim 10^{15}$ M$_\odot$) at $z\gt 0.5$, which act as powerful lenses to study galaxies in the very distant Universe. We also provide first assessments of the evolutionary state, galaxy populations, and large-scale environment of eMACS clusters and release to the community the cluster sample and supporting spectroscopic catalogues to further the understanding of the first generation of truly massive clusters to have formed in the Universe.