Sweeney Todd is a fictional villain immortalised in the Broadway play "Sweeney Todd, the Demon Barber of Fleet Street" by Stephen Sondheim. The play is based on folklore dating back to the 19th Century and is set in London.
Sweeney Todd is a barber who cuts the throats of gentlemen whilst shaving them with a straight razor and uses a trapdoor to send the bodies downstairs to his accomplice. His partner in crime, Mrs Lovett (the "crafty butcher of Fleet Street"), owns the shop below and bakes the poor victims into meat pies. Eventually, both are tried for their crimes and hanged.
It is sometimes claimed that the Sweeney Todd story is based upon fact, but no evidence of this has ever been found. It is far more likely that the play was based on "penny dreadful" horror stories sold in Victorian times.