Zone

is a one-shot manga written by Katsura Hoshino that

introduced the main concepts of D.Gray-man. Due to the positive reception it received she began work on the latter.

Differences with D.Gray-man
There are several differences between Zone and D.Gray-man:
 * Akuma are created by one signing a contract instead of reviving the dead.
 * The main character was not Allen but a similar character named Robin who instead of being cursed by an Akuma was an Akuma herself converted by Cross by having Innocence grafted to her.
 * Robin wasn't born with her Anti-Akuma Weapon on her arm; it was surgically implanted onto her body, the arm having originally belonged to Cross.
 * Lenalee is not member of the Black Order but instead a waitress at a Chinese restaurant in Japan, and is half Japanese.
 * The Earl hasn't taken control of Japan, and it is free from his influence for the most part.
 * The Black Order headquarters are located somewhere in France instead of England.
 * Akuma sport horns on their foreheads, reminiscent to the oni creatures from Japanese mythology.
 * An akuma's creation can be fully stopped only if the person who signed the Earl's contract rips it apart himself or herself.
 * The ritual to create the Akuma can only be completed when the Earl himself stabs the unsuspecting contractee with a broadsword, then he uses the victim's blood to incite the Akuma to start moving. Finally, the Akuma itself absorbs the soul, flesh, and blood of the deceased contractee taking on his or her appearance.