"Massive black holes (MBHs) weighing million solar masses and above inhabit the centers of today's galaxies, weighing about a thousandth of the host bulge mass. MBHs also powered quasars known to exist just a few hundred million years after the Big Bang. Owing to observational breakthroughs and remarkable advancements in theoretical models, we do now that MBHs are out there and evolved with their hosts, but we do not know how they got there nor how, and when, the connection between MBHs and hosts was established.To have a full view of MBH formation and growth we have to look at the global process where galaxies form, as determined by the large-scale structure, on Mpc scales. On the other hand, the region where MBHs dominate the dynamics of gas and stars, and accretion occurs, is merely pc-scale. To study the formation of MBHs and their fuelling we must bridge from Mpc to pc scale in order to follow how galaxies influence MBHs and how in turn MBHs influence galaxies.BLACK aims to connect the cosmic context to the nuclear region where MBHs reside, and to study MBH formation, feeding and feedback on their hosts through a multi-scale approach following the thread of MBHs from cosmological, to galactic, to nuclear scales. Analytical work guides and tests numerical simulations, allowing us to probe a wide dynamical range.Our theoretical work will be crucial for planning and interpreting current and future observations. Today and in the near future facilities at wavelengths spanning from radio to X-ray will widen and deepen our view of the Universe, making this an ideal time for this line of research."