"* Some great leaders and minions, from the evil mastermind to the lower lackeys. Stats are a bonus!"
If politics is a factor in your game (various factions what not) have the leaders of the two strongest both be members of this organization. I love the idea mentioned earlier of people who pretend to be undead so how about this: Landowners leader the Duke of Westford has not been seen outside in years, he meets everyone in his club, a typical stuffy political organization where the members of his faction sit around drinking and playing games. He runs everything from his private room. Let him be the aforementioned vampire wannabe. The PCs only see hisl ackeys, but hear all manner of stories about him. Enter the leader of the second faction, the Merchants, Reginold Woolstead. He is popular, public and perhaps a potential ally of the PCs. Both men have very obvious political machines and a very bitter rivalry for the attention of the King. But in reality they both are members of the organization. They public political manuverings are really just a show to place Club members in important posts, and otherwise advance the power of the Club w/o raising any suspiscion. Eventually any potential threats are either co-opted or elimnated by one or the other.
* Devious Plots and Methods of Spreading Terror.
Other than political manuverings, which can be scary enough. The Club wants any uncontrolable group eliminated. A typical tactic is to kidnap a family member of an enemy, turn them into a mindless undead, and then leave them wandering some place obvious. This cause horror to the family. Create Zombies that can pass for the living with a little effort on the creators part, have these appear in public, be recognized, and then do soemthing discraceful, the deadly the better, and bring shame on their families.
The creation of Mobs of mindless undead, placed randomly, can fuel a demand from the citizinry for more protection. The Club fills the ranks of the city guard with their own people, who know nothing but whose loyalty is well bought or coerced. When the king eventually dies they have that much more control over the capitol.
Maybe the reason they aren't lead by a lich is that none of them know how to become a lich. They are desperately seeking that information. In the mean time they have come up with some pretty gruesom rituals: eating the flesh of undead, bathing in blood, etc.
The Club also has several lesser cults and organizations of the more cliched type. These serve as fronts and pawns for the Inner Circle. Each one think it is the only real cult in the kingdom, and the various groups may actually wage war against one another. Causing more chaos which plays into the Clubs plans.
* Hideouts and meeting places.
The crypt of the kings, that no one should be able to enter except the King's Key Keeper, who just happens to be in the Club. If you've read/seen LotR you know what kind of place I mean.
Respectful homes dotting the country side. Where better to raise an army of undead than on the estate of some lord that is far removed from prying eyes?
The Dead Grove. In the perpetually closed Royal Conservatory, a vast greenhouse built by the last queen, members of the Club have been cultivating a new type of undead: skeletal treants and undead dryads. Let your treant player deal with that.
* Possible sub-motives or detailed ideas.
While the Club seems unified there are actually several motives of the inner circle. One wants to replace the king with a puppet and rule behind the throne, one wants to become a lich and live forever, one is making a fortune off the various sub cults, one may actually worship death (the real sicko of the group), etc. Money and power are big factors for these people. They may see it as largely just another social club to make connections and win favors. They all think they are using the others, and all of them think their goals are the only real ones. If you want a hook for the next story line have one of them be an agent of some Demon Lord/Yuan-ti/Mindflayer who wants to soften the kingdom up for takeover.
* Defenses against pesky heroes.
Buy them. Simple enough. They have lots of public faces, one of them will be appealing to the PCs as a patron. If they get to close to the truth, set them on the trail of one of the lesser cults. If it gets really bad, the Club will set one of the Inner Circle up to take the fall, making him out to be the master mind. When the PCs think they have won, it keeps going.
If the PCs have fleshed out backstories that may come into play: blackmail, kidnapping, extortion, etc. are easy tricks to fall back on.
* Possible Allies, like local orc tribes
Union bosses are better than orcs. They may not show up on "detect evil." Barring other allies with in the society you might have a group of out cast druids and their elven followers, who have taken the first of many steps to become drow-like w/o being drow. If you are looking for races, some simple brutish group like orcs works, but maybe try gnolls instead, whose fear of a hyenia like death godess can be manipulated by the Club.
Kobolds are great tunnelers, perfect to send to excavate those ancient lairs that may contain necromantic secrets. If you think they find somethign, then the Club sends those pesky adventurers to take it away from them.
