Forgeries are artificial Deadmen created by Tamaki to serve his purposes in several experiments.
Forgeries are an experiment in Deadman Wonderland that Tamaki Tsunenaga had been planning with the help of Rei Takashima. Their overall purpose was to replicate the abilities of the deadmen that reside in the prison. They were able to do this by finding compatible blood types and mixing it in with shards of the red diamonds found over Tokyo after the Great Tokyo Earthquake as well as from the stone embedded in Ganta's chest. The results were a success as they were able to create artificial deadmen, hence the name Forgery.
All Forgeries use the same Branch of Sin, which allows their blood to turn into snake like creatures, injecting a poison into their victims bodies that causes the skin to swell up around the puncture wound and burst, killing the victim in a brutal manner. They all wear black robes to hide their bodies and wear identical masks. It is revealed that the masks are a key component as they release an ecstasy-like drug that makes the user want to wear the mask, helping them forget their identity and making killing acceptable.
The first forgeries were created after information leaked to the outside world about the brutal tryant that Tamaki was as well as the treatment of the deadmen prisoners. Tamaki is able to falisfy the information regarding deadman by pitting Azami Mido, who was turned into Forgery, against a prisoner on death row. The brutal display of carnage turns public opinion against the deadmen as Tamaki arranges for several matches against the deadmen after sending them brutal reminders about their traumatized pasts. Ganta tries to fight back against his fellow deadmen, knowing that Azami is one of them as he tries to save her.
After fighting several forgeries, Ganta is able to save Azami and works with Makina to try and destroy the emitter that allows Tamaki and Rei to control the masks of the Forgeries. However, they find that they had moved the transmitter and that most of the Forgeries are dead due to the attacks by the prison security and the deadman, including Azami who was killed by Shiro.