  We propose the formation of massive pristine dark-matter (DM) halos with
masses of $\sim 10^8~M_\odot$, due to the dynamical effects of frequent mergers
in rare regions of the Universe with high baryonic streaming velocity relative
to DM. Since the streaming motion prevents gas collapse into DM halos and
delays prior star formation episodes, the gas remains metal-free until the halo
virial temperatures $\gtrsim 2\times 10^4~{\rm K}$. The minimum cooling mass of
DM halos is boosted by a factor of $\sim 10-30$ because frequent major mergers
of halos further inhibit gas collapse. We use Monte Carlo merger trees to
simulate the DM assembly history under a streaming velocity of twice the
root-mean-square value, and estimate the number density of massive DM halos
containing pristine gas as $\simeq 10^{-4}~{\rm cMpc}^{-3}$. When the gas
infall begins, efficient Ly$\alpha$ cooling drives cold streams penetrating
inside the halo and feeding a central galactic disk. When one stream collides
with the disk, strong shock forms a dense and hot gas cloud, where the gas
never forms H$_2$ molecules due to effective collisional dissociation. As a
result, a massive gas cloud forms by gravitational instability and collapses
directly into a massive black hole (BH) with $M_\bullet \sim 10^5~M_\odot$.
Almost simultaneously, a galaxy with $M_{\star, \rm tot}\sim 10^6~M_\odot$
composed of Population III stars forms in the nuclear region. If the typical
stellar mass is as high as $\sim 100~M_\odot$, the galaxy could be detected
with the James Webb Space Telescope even at $z\gtrsim 15$. These massive seed
BHs would be fed by continuous gas accretion from the host galaxy, and grow to
be bright quasars observed at $z\gtrsim 6$.
