The Host

The Host was a human boy who lived in 19th century Scotland.

Biography
The Brethren took the boy away from his homestead in the night and allowed him to be bitten by the Lupine Wavelength Haemovariform they worshipped. He became infected by the Haemovariform that "sat in his heart and devoured his soul". The Host was given all-black eyes and was entirely controlled by the Haemovariform, which could access the Host's memories and use some basic Telepathy. Every full moon, he would transform into a true Haemovariform. The real boy was long dead. When the Tenth Doctor used the Torchwood House telescope to destroy the Haemovariform, the Host began to live again. He asked the Doctor to increase the effect and destroy him, which he did. The Host died sacrificing his life to save the world. (DW: Tooth and Claw)