What's the meaning behind this quote? “Percival Wemys Madison sought in his head for an incantation that had faded celan away.” (Chapter 12 pg. 201)

In this quote, Golding illustrates the boys' loss of identity through the character of Percival. At the start of the novel, Percival repeatedly recited his name.... his full name, from what we can infer is a way to remember who he is.... his identity.

In Chapter Twelve, Percival cannot bring his name to mind, which tells us that over the course of the novel, he lost both his full name and his address. He is no longer Percival Wemys Madison, he is the member of a tribe, who does as he's told, who eats and sleeps.