Long ago, The assassin’s guild was hired to kill of The Evil One of Keldor(Litch or suitably powerful Evil thing). They sent three assassins on this apparent suicide mission. On their way to the Evil One’s lair, they encountered a paladin. They had cleverly disguised themselves as normal adventurers, so he suspected nothing. He was questing to destroy “Evil” so they helped steer him towards The Evil One’s lair.
Of course the Paladin, with his forces of light and goodness defeated The Evil One of Keldor.
The three assassins were both amused and impressed. They found out he was very useful in killing certain targets (notably big, powerful, and EVIL). These three became his handlers, feeding him intel on various targets, and passing wealth -minus the guild fees- on to him. Discovering a very effective racket, the three assassins began to pose as “agents of a secret order”. They began to recruit “good” paladins and clerics (as well as others of stiff moral bent) to take on contracts that the guild could not handle.
Eventually, they created “The Order of the Dry Bones”. It was good for recruitment and organization. The assassin’s guild passes information to various Order Houses, who then assign the quests to various “Good Groups”. The Contract Fees pay for the upkeep of The Order Houses and fill all the various member’s Assassin Guild Fees.
Sometimes, when the Order wants to go kill a Great Evil, the assassins will shop around and find someone who is willing to pay for the contract upon the Great Evil. That way the Order continues to make money for the Guild.
None of these “good” people really know who are their “Aid in their battle against Evil” is.