Samantha's Diary was written by Samantha Gilbert in 1912. She wrote about her strange activities.
Elena and Matt found Samantha's diary in Meredith's apartment, hidden inside her closet. It is unknown how Meredith came into the possession of the diary, or whether it was stolen or went into the possession of the Fell family at some point in time. Matt brought it with him, hoping they could find something in it. Since it's a Gilbert Diary, Matt told Elena she must have it with her. Samantha had mentioned about the murders that took place in 1912, and said that "she doesn't feel like herself". Later, Samantha was shown as the Serial Killer in 1912.
1912: "I don't feel like myself. I am losing time... As though I am going mad".
Samantha's handwriting style seems to shift differently in different lines of her diary, from the direction of a slant, to cursive design. According to the Principles of Graphology, a person's mood or mindset affects their handwriting style. This could possibly be a reference that not all of the entries in the diary were necessarily written by the same personality of Samantha and that both the real Samantha and the Darkness could have written entries in the book.