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We search for black holes across the entire history of the Universe from its earliest epochs to the present day. We focus on those with the highest masses (greater than one billion solar masses) and fastest growth rates. The origin of such black holes can presently not be explained. We identify candidates from a range of surveys including SkyMapper, Gaia and WISE. We confirm their nature with observations at the ANU 2.3m telescope among others, and aim to obtain the most complete demographics of early black holes that is possible. Finally, we measure the masses of the black holes from NIR spectroscopy at very large telescopes. Observing the properties of these black holes such as their brightness variability over months and years or their spectral energy distribution reveals properties of their accretion disks and helps with calibrating the observed masses. We also study black holes that rapidly switch their accretion on and off due to unknown mechanisms. The end goal of the project is to understand the demographics and physical conditions of black hole growth.