The Others are hatching. One of them splits open Calvin Riddle's head and slides out of him like a calf through the birth canal. It looks like Calvin. It sounds like Calvin. it isn't Calvin.
Calvin survived though. Calvin persisted. Though stunned to discover he had been an incubator for this... thing, Calvin is gathering strength. Soon, he will be ready to fight.
Split is the first book in "The Other" series. Aliens, mutations, or the vision of madmen? The answer depends on the perspective.