  The dominant mechanism forming multiple stellar systems in the high-mass
regime (M$_\ast \gtrsim $ 8 $M_{\odot}$) remained unknown because direct
imaging of multiple protostellar systems at early phases of high-mass star
formation is very challenging. High-mass stars are expected to form in
clustered environments containing binaries and higher-order multiplicity
systems. So far only a few high-mass protobinary systems, and no definitive
higher-order multiples, have been detected. Here we report the discovery of one
quintuple, one quadruple, one triple and four binary protostellar systems
simultaneously forming in a single high-mass protocluster, G333.23--0.06, using
Atacama Large Millimeter/submillimeter Array high-resolution observations. We
present a new example of a group of gravitationally bound binary and
higher-order multiples during their early formation phases in a protocluster.
This provides the clearest direct measurement of the initial configuration of
primordial high-order multiple systems, with implications for the in situ
multiplicity and its origin. We find that the binary and higher-order multiple
systems, and their parent cores, show no obvious sign of disk-like kinematic
structure. We conclude that the observed fragmentation into binary and
higher-order multiple systems can be explained by core fragmentation,
indicating its crucial role in establishing the multiplicity during high-mass
star cluster formation.
