Megiddo is a mysterious organization from Splinter Cell: Convition, mentioned only in the SMI Database in Splinter Cell: Blacklist, and possibly indirectly or refered to in H.A.W.X.
They're a mysterious entity with vasts amount of power and influence in the world. The organization is highly clandestine, with only a handful of people knowing about their existance. They go to very long lengths to keep their existance a secret, which does include killing their own people. For example, Lucius Galliard, an associate of Megiddo, had been shot by an assassin working for the organization to keep Sam Fisher from discovering them, in which the shooter's car was rigged and exploded when he entered it before Sam could catch him.
The organization is responsible for the actions carried out by Tom Reed, as he was sent by them to kill President Caldwell for not matching her policies with theirs, in which this failed.
Fire Rebellion Storyline
Tom Reed will appear in Season 9, in which Megiddo sent him to kill Princess Celestia. This is one of the reasons Dimitria came to the heroes' aid.