Both groups are trying to end the Withering, actually, though they're going about doing so differently. But I know which one you mean.

As for the monster that sacked Nemhim City... well, that's for the characters to decide. They are free to make whichever choice they feel that they need to in a rather literal sense, and turning their focus to a new threat such as the creature or ignoring it are both possibilities. Every decision in The Prophecy has consequences, however, and I don't think I'm spoiling anything that people haven't figured out already when I say that this menace isn't very likely to stop at Nemhim. The longer it exists the more it will kill, and the stronger it will become.
The problem is that I don't actually think that the characters would be able to face it as they are now; not without help, at least.