  Around a million quasars have been catalogued in the Universe by probing
deeper and using new methods for discovery. However, the hardest ones to find
seem to be the rarest and brightest specimen. In this work, we study the
properties of the most luminous of all quasars found so far. It has been
overlooked until recently, which demonstrates that modern all-sky surveys have
much to reveal. The black hole in this quasar accretes around one solar mass
per day onto an existing mass of $\sim$17 billion solar masses. In this process
its accretion disc alone releases a radiative energy of $2\times 10^{41}$
Watts. If the quasar is not strongly gravitationally lensed, then its broad
line region (BLR) is expected to have the largest physical and angular diameter
occurring in the Universe, and will allow the Very Large Telescope
Interferometer to image its rotation and measure its black hole mass directly.
This will be an important test for BLR size-luminosity relations, whose
extrapolation has underpinned common black-hole mass estimates at high
redshift.
