The Twist Ending (situational irony)
Like many twist endings, the reveal that Teddy Daniels is (or might be) Andrew Laeddis functions as situational irony, a sudden reversal that reveals something unexpected to the reader about the character. Throughout the novel, Teddy see himself as the one in charge, the one who is able to make others talk, and the one who must shoulder the burden of exposing Ashecliffe and avenging his wife. The twist upends our perception of the character by revealing that in fact, it is Dr. Cawley and Dr. Sheehan who are trying to solve the mystery of his trauma, and how it can be cured.
