There are some Rise of the Runelords spoilers ahead, as well as the obvious Shattered Star spoilers, so be warned.
I recently finished GMing Rise of the Runelords for my group, and have since moved on to GMing Shattered Star. I've been trying to spice up Shattered Star by really playing up the Pathfinder Society portion; I've been making plans to add in a bunch of Pathfinder Society scenarios as optional side-missions to be played between (and sometimes during) the main books of Shattered Star.
However, in my attempt to personalize this playthrough of Shattered Star, I had an idea: see, when my players fought Karzoug on the last session of Rise of the Runelords, they lost. I've issued forth the statement that, despite this, their characters in Shattered Star are in the same continuity as the ones in Rise of the Runelords, but what I haven't done is tell them how Karzoug was defeated so that the world could go on as normal. In fact, I believe that the only time it was brought up, I had Sheila Heidmarch state that it was "a long story."
That brings me to my idea: I'm thinking that Karzoug wasn't defeated, by either their old group of adventurers, another group, or any other means. Instead, he decided to bide his time, and to ensure that when the other Runelords decided to rouse from their slumber, he would be the most powerful one, and be able to dispatch them. How did he decide to do this? Why, by assembling the fragments of the Shattered Star, and rebuilding the Sihedron, of course.
Now, this is where the plot gets loopy. Instead of using his sheer arcane power to find the Shards, what if he decided that it wasn't worth his time to do so directly? What if, instead of searching for the Shards himself, he managed to convince a group of lowly adventurers to find them for him?
What I'm sort of planning is thus: Karzoug, undefeated by Varisia's greatest heroes, wants to ensure that his rise - and that of his empire - goes completely unchallenged. To do this, he plans on re-assembling the Sihedron, which has the added benefit of bringing back the Dead King Xin, who Karzoug would certainly love to put down once and for all. To this end, he decides to masquerade as someone who could, within reason, be able to bring on small groups of adventurers to do his bidding without any questioning. And who better than the lovely Sheila Heidmarch?
Thus, Karzoug kills / captures / whathaveyous the real Sheila Heidmarch, disguising himself as her (I know he can't use Illusion spells, but he's rich. He can afford something). He spends a couple of years establishing himself as the "real" Sheila, and searching for the location of the first Shard to kick off his little game, whereupon he brings in the PCs and the campaign starts.
What I'm asking, then, is does this sound like a good idea? If so, how should I go about alluding to it during the campaign (and should I even bother doing so)? How should I have it play out in the end? I was thinking about having "Sheila" congratulate the PCs after they put down Xin, asking for the Sihedron, and then revealing herself as Karzoug (maybe teleporting the group to the Eye of Avarice), whereupon the PCs would have to fight him, but I don't know whether or not that's a good idea.
So, comments, critiques, suggestions? Anything's appreciated!