  H1821+643 is the most X-ray luminous non-beamed AGN of $L_\mathrm{14-150
keV}= 5.2\times 10^{45}$ erg s$^{-1}$ in the Swift/BAT ultra-hard X-ray survey
and it is also a hyper-luminous infrared (IR) galaxy $L_\mathrm{IR} = 10^{13.2}
L_\odot$ residing in the center of a massive galaxy cluster, which is a unique
environment achieving the rapid mass assembly of black holes (BH) and host
galaxies in the local universe. We decompose the X-ray to IR spectral energy
distribution (SED) into the AGN and starburst component using the SED fitting
tool CIGALE-2022.0 and show that H1821+643 consumes a large amount of cold gas
($\dot{M}_\mathrm{con}$) with star-formation rate of $\log (
\mathrm{SFR}/M_{\odot}~\mathrm{yr}^{-1}) = 3.01 \pm 0.04$ and BH accretion rate
of $\log (\dot{M}_\mathrm{BH}/M_{\odot}~\mathrm{yr}^{-1}) = 1.20 \pm 0.05$.
This high $\dot{M}_\mathrm{con}$ is larger than the cooling rate
($\dot{M}_\mathrm{cool}$) of the intra-cluster medium (ICM),
$\dot{M}_\mathrm{con}/\dot{M}_\mathrm{cool} \gtrsim 1$, which is one to two
order magnitude higher than the typical value of other systems, indicating that
H1821 provides the unique and extreme environment of rapid gas consumption. We
also show that H1821+643 has an efficient cooling path achieving from $10^7$ K
to $10^2$ K thanks to [OIII] 63 $\mu \mathrm{m}$, which is a main coolant in
low temperature range ($10^4$ K to $10^2$ K) with a cooling rate of
$\dot{M}_{\mathrm{cool}}=3.2\times 10^5\ M_{\odot}\mathrm{~yr^{-1}}$, and the
star-forming region extends over 40 kpc scale.
