  We present the results of the first fully cosmological hydrodynamical
simulations studying the merger-driven model for massive black hole (BH) seed
formation via direct collapse. Using the zoom-in technique as well as particle
splitting, we achieve a final spatial resolution of $2$ pc. We show that the
major merger of two massive galaxies at redshift $z \sim 8$ results in the
formation of a nuclear supermassive disk (SMD) of only $4$ pc in radius, owing
to a prodigious gas inflow sustained at $100$-$1000$ $M_{\odot}$ yr$^{-1}$. The
core of the merger remnant is metal-rich, well above solar abundance, and the
SMD reaches a gaseous mass of $3 \times 10^8$ $M_{\odot}$ in less than a
million years after the merger, despite a concurrent prominent nuclear
starburst. Dynamical heating as gas falls into the deepest part of the
potential well, and heating and stirring by supernova blastwaves, generate a
turbulent multi-phase interstellar medium, with a gas velocity dispersion
exceeding 100 km s$^{-1}$. As a result, only moderate fragmentation occurs in
the inner $10$-$20$ pc despite the temperature falls below $1000$ K. The SMD is
Jeans-unstable as well as bar-unstable and will collapse further adiabatically,
becoming warm and ionized. We show that the SMD, following inevitable
contraction, will become general relativistic unstable and directly form a
supermassive BH of mass in the range $10^6$-$10^8$ $M_{\odot}$, essentially
skipping the stage of BH seed formation. These results confirm that mergers
between the most massive galaxies at $z \sim 8$-$10$ can naturally explain the
rapid emergence of bright high-redshift quasars.
