Nathaniel Chiltern was the insane twin brother of Sebastian Chiltern.
To try to cure him, Sebastian stole Micah Scale's temporal interferometry machine. He wanted to project Nathaniel into the future to a time when there was an antidote to his madness. Instead, it splintered Nathaniel temporally. One of the parts which remained in the 19th century locked Sebastian away in a sanatorium, since his claims about what had happened to his brother seemed lunacy. He later killed his brother for the way he had been treated.
Another splinter of Nathaniel was able to pull the parts that were in the future, around 1957, back to itself. Unfortunately, it also pulled some objects near those fragments along as well. This meant that there were two splinters in the 19th century. One was completely human, whilst the other had one of his eyes replaced by a rodent's mouth, a living worm in place of the little finger on his left hand, a toaster growing out of his back with a trailing electrical cord, and an always-blooming rosebush in place of his left leg. He also implied to the Eighth Doctor that there had been alterations to his genitalia as well.
After recovering all the parts of the temporal interferometry machine, he tested it by sending Constance Jane through it. It succeeded in undoing her mental fragmentation, reuniting her personality and not causing a paradox. The Doctor entered the machine to destroy it, since he knew how dangerous it could be to the universe. His plan worked, even though Nathaniel tried to stop him once he realised what the Doctor intended. Elizabeth Kelly stopped Nathaniel, who killed her. Sabbath snapped his neck in retribution, and since both fragments were still connected, the fully human Chiltern dropped dead at the same time. (PROSE: Camera Obscura)