I was idly wondering what the Rival Adventurers were up to during the events of 3FoE, when it occurred to me that the 3 Adventurers themselves were thematically similar to the 3 sections of the adventure. Auric suits the open combat of the Hextor citadel, Tirra would excel at sneaking and climbing around in the caves, and Khellek would be interested in the evil magic of the Vecna sanctum.
My thought was that each section of the dungeon could contain one of the 3 adventurers, stranded or held prisoner. Once discovered, they would provide a little extra muscle for the PCs, as well as some motivation to find the other two. Either way, the Rivals would resent being rescued by their inferiors, but would hide that fact until safely out of the dungeon.
What got me started was the description of the Hextor Battle Temple, where "the faithful and heretics alike fight to the death for glory, honor, and the blessings of the Scourge of Battle." This must happen frequently, since it's the central feature of the citadel, and designed for an audience. Perhaps Hextor requires a monthly "sacrifice" in the form of the losing combatant. Since "heretics" are also fighting, it implies that the Hextorites are recruiting from Diamond Lake. Otherwise, the population of the citadel would dwindle pretty fast.
Auric, after some drunken bragging about his Champion's Belt, was quietly approached by one of the cultists as being worthy of invitation to the Battle Temple. The gladiator jumped at the chance, especially after the disappointing lack of action at the Stirgenest Cairn. After winning the battle, he was given the choice of joining the cult or being sacrificed himself. Auric wisely chose to pose as a petitioner, to give Tirra and Khellek time to realize he was missing and come looking.
Tirra and Khellek investigate, and find leads to the Dourstone, but don't specifically know about the Hextorites. They hire some dumb muscle from the Feral Dog, and infiltrate the mine. Then they separate, either willingly or through mishap; Tirra into the caves and Khellek into the labyrinth. All the mercenaries get killed, stranding the two Rivals. Khellek has used up all his spells surviving the kenku and tricking them into thinking he's dead or escaped. He's now hiding in the Storage Chamber, hoping to rest up enough to reacquire some spells. Tirra is stuck in the lower passage leading to area 16, trapped between the archers and the chokers, wondering when her Darkvision spell is going to wear off...
Maybe Overload or the next chapter will tell us more about the Rivals, and completely contradict this idea. In that case, you could use 3 other NPCs in whom the PCs have a strong interest. Or perhaps one of the PCs themselves gets lured alone to the Battle Temple, and the rescue is what motivates the intial attack. For that matter, getting a PC invited to the battle would be an excellent way to initiate an infiltration. Any other ideas?