The Spiderweb protocluster.

Protoclusters are defined as overdense regions in the high redshift Universe which are expected to evolve into massive, virialized clusters of galaxies at the present epoch. Finding and characterizing protoclusters is key to study the large scale structure of the Universe and the transformational processes that affect star formation and nuclear activity in the member galaxies. In order to confirm their nature and to trace the activity of several processes occurring during their rapid evolution, protocluster candidates must be followed-up with multi-wavelength campaigns. One of the few protocluster intensively studied in the last 25 years is the archetypal Spiderweb Galaxy which lies at the center of a protocluster at $z=2.16$. Particularly, thanks to X-ray data we are able to study the nuclear emission from the central galaxy and protocluster members and the diffuse emission of the proto-intracluster medium, revealing an enhancement of nuclear activity and, for the first time, the presence of a strong cool core in a halo at $z$ > $2$. These results have profound implications for the formation and evolution of protoclusters in the Universe.