Complex sentence: Moments later, Pappu and Sandy join O.K., whereupon they realize that the figure who challenged Mukesh is Shanti Priya's ghost, who smiles at them, runs up the stairwell, and disappears.
Split into simpler sentences: Moments later, Pappu and Sandy join O.K., whereupon they realize that the figure who challenged Mukesh is Shanti Priya's ghost. As they realize this, the ghost smiles at them, runs up the stairwell, and disappears.