S4N-T was a robot created by Franken after he came into posession of a telepathic circuit during the Christmas period.
After Adam ripped off Franken's research, Franken sent S4N-T to kill him, and he succeeded. However when S4N-T said he would kill Stevenson, Franken told him not to. Still, S4N-T went and did it anyway.
Franken and S4N-T then tracked down Reginald and killed him too. The Beginner was already there waiting for them with Brody, and disarmed Franken and killed him, causing S4N-T to go into a state of panic and grow to be a giant. The Beginner used his TARDIS to break into his giant body and destroy the telepathic circuit, causing S4N-T to explode. (Red Christmas)