  We report the first identification of a compact dense galaxy overdensity at
$z\sim8$ dubbed A2744z8OD. A2744z8OD consists of eight $Y$-dropout galaxies
behind Abell 2744 that is originally pinpointed by Hubble Frontier Fields
studies. However, no studies have, so far, derived basic physical quantities of
structure formation or made comparisons with theoretical models. We obtain a
homogeneous sample of dropout galaxies at $z\sim8$ from eight field data of
Hubble legacy images that are as deep as the A2744z8OD data. Using the sample,
we find that a galaxy surface overdensity value of A2744z8OD is very high
$\delta\simeq130$, where $\delta$ is defined by an overdensity in a small
circle of $6"$ ($\simeq30$ physical kpc) radius. Because there is no such a
large $\delta$ value reported for high-$z$ overdensities to date, A2744z8OD is
a system clearly different from those found in previous high-$z$ overdensity
studies. In the galaxy+structure formation models of Henriques et al. (2015),
there exist a very similar overdensity, Modelz8OD, that is made of eight model
dropout galaxies at $z\sim8$ in a $6"$-radius circle. Modelz8OD is a progenitor
of a today's $10^{14}M_\odot$ cluster, and more than a half of the seven
Modelz8OD galaxies are merged into the brightest cluster galaxy (BCG) of the
cluster. If Modelz8OD is a counterpart of A2744z8OD, the models suggest that
A2744z8OD would be a part of a forming cluster core of a today's
$10^{14}M_\odot$ cluster that started star formation at $z>12$.
