  The protoclusters in the epoch of reionization, traced by galaxies
overdensity regions, are ideal laboratories for studying the process of stellar
assembly and cosmic reionization. We present the spectroscopic confirmation of
the core of the most distant protocluster at $z = 7.88$, A2744-z7p9OD, with the
James Webb Space Telescope NIRSpec integral field unit spectroscopy. The core
region includes as many as 4 galaxies detected in [OIII] 4960 \AA\ and 5008
\AA\ in a small area of $\sim 3\arcsec \times 3\arcsec$, corresponding to
$\sim$ 11 kpc $\times$ 11 kpc, after the lensing magnification correction.
Three member galaxies are also tentatively detected in dust continuum in
Atacama Large Millimeter/submillimeter Array Band 6, which is consistent with
their red ultraviolet continuum slopes, $\beta \sim -1.3$. The member galaxies
have stellar masses in the range of log($M_{*}/M_{\rm \odot}$) $\sim 7.6-9.2$
and star formation rates of $\sim 3-50$ $M_{\rm \odot}$ yr$^{-1}$, showing a
diversity in their properties. FirstLight cosmological simulations reproduce
the physical properties of the member galaxies including the stellar mass,
[OIII] luminosity, and dust-to-stellar mass ratio, and predict that the member
galaxies are on the verge of merging in a few to several tens Myr to become a
large galaxy with $M_{\rm *}\sim 6\times10^{9} M_{\rm \odot}$. The presence of
a multiple merger and evolved galaxies in the core region of A2744-z7p9OD
indicates that environmental effects are already at work 650 Myr after the Big
Bang.
