The Trapezium Cluster in Orion teaches us that massive stars often form together in compact systems—but how such dense OB groupings are assembled remains an open question. In this talk, I will present high-resolution observations with the Atacama Large Millimeter/submillimeter Array (ALMA) that reveal an exceptionally young, deeply embedded massive quadruple OB system in the making. The four massive protostars are arranged in an off-center belt at radii of 0.02–0.06 pc. By decomposing the ionized and neutral gas components, we trace the dynamic interplay between stars and gas down to thousand-AU scales. The gas kinematics and orbital energetics provide direct evidence for how material transitions from unbound, parabolic trajectories to bound orbits, assembling onto the forming protostars. These observations offer a rare glimpse into the early dynamical assembly of a Trapezium-like massive cluster.
