Complex sentence: The day of the performance comes, and everyone who attends is shocked to find out that not only Terrance has become morbidly obese, but Phillip is not with him; instead, there is a stand-in performs with him, much to everyone's dismay.
Split into simpler sentences: The day of the performance comes, and everyone who attends is shocked to find out that not only has Terrance has become morbidly obese, but Phillip is not with him. Instead, there is a stand-in who performs with him, much to everyone's dismay.