The Assassin profession is only available in the Factions campaign, because that's how they roll. Prophecies and Nightfall users will not be able to use this profession unless they have linked a Factions key to their account, because Assassins are special like ritualists and they deserve their own campaign. Factions owners may only create an Assassin by starting their character in the Factions campaign, as they aren't in any other campaign. They train their Kung Fu with Togo and his minions, er Masters.