Sometimes it just depends on how much campaign prep time you have: the -real- threat organization in my game is a mostly original creation, but I resorted to a published adventure to get my campaign off the ground in a hurry. The Illuminated were the bads in it, and I didn’t have time to change it. So I ran it as written, and absorbed the Illuminated into the larger, more mysterious threat in the process (and that power play became part of the story as well).

And if you have creative players who can come up with organizations, you might as well milk that too…!

I used to worry about bugbears like “continuity” and “metaplot,” which was why I was, at first, angry when they started detailing the Night Masks (for example). I’m much more comfortable with making things my own now, but I still hear those voices nagging me sometimes…

But I think an organization of your own avoids correcting/explaining/divulging:

1) how your interpretation of published material may differ from canon, the DM’s priviledge to change anything, notwithstanding. (Invite players over for a jaunt through the Realms, then have the unavoidable problem of preconceived notions about the setting based on what they’ve read.) With something of your own, you avoid some of that.

2) the mysteries of your creation. Roleplay is an exercise in discovery. I think at its simplest, an element of the DM’s creation preserves that sense of mystery, and as the players romp in the DM’s adventure, that mystery is revealed only as the PCs discover certain elements, or the organization can be crafted to meet the backgrounds/goals of players.

That second is meant to be a reward to the players. In addition to offering something tangible the PCs can grab hold of, the DM asserts his “ownership” of the setting by introducing something of which he reveals as the the story dictates — rather than what can be unearthed in the sourcebook.

I’m not contrary toward published settings as all. In fact, I prefer them, for a host of reasons. I think establishing a mindset where the DM is in “control” of the setting is important. And as a write that, I know there will be a host folks who say such a statement is illusory, but contary to cooperative game play. But that’s what experience has taught me.

Now, of course, the Night Masks have been described in excruciating detail in various sourcebooks. Were I to return to Westgate for a game – and I would in a cold minute, it’s a great setting – I’d use my version of the Night Masks, since I’ve already done the hard work, and they’re supposed to be a secretive organization, so campaign setting geeks like Idran and myself really shouldn’t have anything to complain about. ; )

The Illuminated in Planescape basically appear in one adventure, and the references to them in all the other books would fit in this sentence. So they became the major adversaries of the first act of my campaign, and it’s worked out beautifully.

But then again, I’m a huge campaign setting geek, among many other types of geekery; if I’m not mastering something, even if it’s a part of a given setting I don’t plan to use any time soon if at all, I still feel like I’m not doing my job.