If they don't Follow the Light (you know the one I'm referring to, I think), then they don't Follow the Light and I'll find another way of helping them advance their story.
Their background is this. The sister of a Zebra Tribe shaman has disappeared into the Big Rubble on a personal spiritual quest. Brother has gone in pursuit and found, through the help of his allied spirits, that she is held in Blind Kings Hill.
Venturing there, he meets-up with a Dark Troll messenger come down from Dagori Inkarth looking for Mistress Korazang of the Sazdorf Clan - the current occupants of Blind Kings Hill. Turns out that the Sazdorfs are awaiting the return of this Mistress Race Troll and discovered that she has been spirtually imprisoned in some way. The Zebra Shamaness has been captured by the same thing, so the PCs are looking for a way to free the two captured priestesses.
All the know is that they have to undertake the Blue Serpent HeroQuest: they don't know what that is, how to do it, or what it concerns. So they've gone looking for answers at the Old Sun Dome...