Mr Sin

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

History
The Peking Homunculus was a 51st century cyborg made in Peking for the commissioner of the Icelandic Alliance. It was made as a toy, a plaything for his 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) It was actually one of many cyborgs created at around that time, mostly for military purposes. (FP: In the Year of the Cat)

It played a key role in history when the pig part took over and almost caused World War VI. After that it disappeared. It later turned out to be in the possession of Magnus Greel, a fugitive war criminal from the fallen Supreme Alliance, who took it back in time with him, to the 19th century and, armed with a knife, 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 started firing at anyone who moved, including Greel's own men and then 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 brain matter acquired from a slaughterhouse. Hsien-Ko believed that she could use Sin as a weapon the way Greel had failed to do so. Although she was able to maintain a tighter hold on Sin's more violent tendencies- disguising him as a child to avoid attracting attention- she eventually lost control when he was damaged in a fight at her base. In the subsequent fight, the Doctor decapitated Sin. K9 later destroyed the robot's head to prevent it ever being reactivated. (MA: The Shadow of Weng-Chiang).