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We report the first identification of a compact dense galaxy overdensity at$z\sim8$ dubbed A2744z8OD. A2744z8OD consists of eight $Y$-dropout galaxiesbehind Abell 2744 that is originally pinpointed by Hubble Frontier Fieldsstudies. However, no studies have, so far, derived basic physical quantities ofstructure formation or made comparisons with theoretical models. We obtain ahomogeneous sample of dropout galaxies at $z\sim8$ from eight field data ofHubble 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 smallcircle of $6"$ ($\simeq30$ physical kpc) radius. Because there is no such alarge $\delta$ value reported for high-$z$ overdensities to date, A2744z8OD isa system clearly different from those found in previous high-$z$ overdensitystudies. In the galaxy+structure formation models of Henriques et al. (2015),there exist a very similar overdensity, Modelz8OD, that is made of eight modeldropout galaxies at $z\sim8$ in a $6"$-radius circle. Modelz8OD is a progenitorof a today's $10^{14}M_\odot$ cluster, and more than a half of the sevenModelz8OD galaxies are merged into the brightest cluster galaxy (BCG) of thecluster. If Modelz8OD is a counterpart of A2744z8OD, the models suggest thatA2744z8OD would be a part of a forming cluster core of a today's$10^{14}M_\odot$ cluster that started star formation at $z>12$.

Very Compact Dense Galaxy Overdensity with {\delta} ~ 130 Identified at z ~ 8: Implications for Early Protocluster and Cluster-Core Formation