  High-redshift quasars ($z\gtrsim6$), powered by black holes (BHs) with large
inferred masses, imply rapid BH growth in the early Universe. The most extreme
examples have inferred masses of $\sim \! 10^9\,$M$_\odot$ at $z = 7.5$ and
$\sim \! 10^{10}\,$M$_\odot$ at $z = 6.3$. Such dramatic growth via gas
accretion likely leads to significant energy input into the quasar host galaxy
and its surroundings, however few theoretical predictions of the impact of such
objects currently exist. We present zoom-in simulations of a massive
high-redshift protocluster, with our fiducial FABLE model incapable of
reproducing the brightest quasars. With modifications to this model to promote
early BH growth, such as earlier seeding and mildly super-Eddington accretion,
such `gargantuan' BHs can be formed. With this new model, simulated host dust
masses and star formation rates are in good agreement with existing JWST and
ALMA data from ultraluminous quasars. We find the quasar is often obscured as
it grows, and that strong, ejective feedback is required to have a high
probability of detecting the quasar in the rest-frame UV. Fast and energetic
quasar-driven winds expel metal-enriched gas, leading to significant metal
pollution of the circumgalactic medium (CGM) out to twice the virial radius. As
central gas densities and pressures are reduced, we find weaker signals from
the CGM in mock X-ray and Sunyaev-Zeldovich maps, whose detection - with
proposed instruments such as Lynx, and even potentially presently with ALMA -
can constrain quasar feedback.
