Mr Sin

The Peking Homunculus, also known as Mr Sin, was a murderous cyborg.

History
The Peking Homunculus was a 51st century cyborg made by Dr. Sa Yy Findecker for Ingrid Bjarnsdottir, the commissioner of the Icelandic Alliance, as a toy for her two children. It contained a series of magnetic fields on a printed circuit, a small computer, and a single organic component - the cerebral cortex of a pig. (DW: The Talons of Weng-Chiang, BFA: The Butcher of Brisbane)

It played a key role in history when the pig part took over and almost caused World War VI, then disappeared. It later showed up in the possession of Magnus Greel, a fugitive war criminal from the fallen Supreme Alliance. He took it with him to the 19th century. Armed with a knife, it did his bidding.

During the final battle between the Fourth Doctor and Greel, Mr Sin, whom Greel had unwisely allowed access to a laser rifle, became trigger-happy and fired at anyone who moved, including Greel's men and Greel himself. The Doctor finally threw it to the ground and tore out its fuse. (DW: The Talons of Weng-Chiang)

Sherlock Holmes later investigated Mr Sin, terming it The Affair of the Walking Ventriloquist's Dummy, but was unable to solve the matter. (NA: All-Consuming Fire)

Sin was later reactivated by Hsien-Ko Chang, the daughter of Greel's former ally, Li H'sen Chang, using the prototype control fuse and brains from a slaughterhouse. Hsien-Ko believed that she could use Sin as a weapon the way Greel had failed to do so. Although she maintained a tighter hold on Sin's more violent tendencies — disguising him as a child to avoid attracting attention — she lost control when he was damaged in a fight at her base. In the subsequent fight, the Doctor decapitated Sin. K9 later destroyed the cyborg's head to prevent it ever being reactivated. (MA: The Shadow of Weng-Chiang).