Complex sentence: Glen wonders if this is the construction worker he had heard was buried in the walls of the house, but before either of the boys can react, the construction worker suddenly wakes up and pulls Terry into the wall, which seals up behind him.
Split into simpler sentences: Glen wonders if this is the construction worker he heard was buried in the walls of the house. Before either of the boys can react, the construction worker pulls Terry into the wall, which seals behind him.