Bingo - the finale is sort of ambiguous, and the origins of the Scar were also left somewhat ambiguous, IIRC. Within the later additions of "The Age of the Wolves", you can certainly find ways to round it up, but by itself, it's not really clear how to explain it to the players/how to continue it.
The concept itself is easy to stylistically place - "Creepers" or, rather "The Creature from the Wolfstaag" in the Shannara novels function more or less like the Scar does:
http://shannara.wikia.com/wiki/Shannara_CreaturesSo, expanding on that concept - say, connecting it to the City of the Gods - is possible. In any case, any reasonable DM will have to write the last chapter of that campaign arc by him- or herself if his goal is to give the players a satisfying conclusion.
Personally, I solved the question for Scar's origin the cheapest way possible, IIRC - it was just a mutated giant spider that had sort of absorbed the armor of several knights that had been unlucky enough to cross its path. A bit cheap, but easy to find out by the players themselves, and rounding up a fairly complex story.
Still so, thirteen years after I first ran this one, RS remains my single favorite BM adventure. :) It's reasonably short, no encounter really plays out like the other, and the main plot, without any meta-setting implications, is dark enough to create some shock-value, which is important when you're dealing, like most of us do, with experienced players. Speaking solely as a fan, I'd preferred this storyline to continue, rather than seeing "just" updates of the classic modules, for most of the remaining ZGG run. Not that they were bad, not at all - but this story was the kind of fresh impulse that the setting needed to move beyond the "narrative spaces" that we'd already come to know.
Aaaah, the memories. :)