Black Widow is a White Supremacist terror organization specializing in assassinations and bombings. They are the new target given the Death Master by the CIA but to take them down, Frank Charon must first become a member.
[Note: the fact that a group of white racists would call themselves 'Black Widow' is, according to Charon, "woefully ironic, something I wouldn't have dreamed [them] capable of".]