I didn't understand this part "She could for example tell them there is an evil druid (Reidoth) impersonating her who will tell them where the cave is." 

Reidoth is impersonating Venomfang? Why? How? 

In my game one of the players was using the pre-gen with the background tied to Thundertree, and wanted to go there to "kill the dragon". The party didn't go there until after they'd already finished the mine, so they were 4th level. The PC with the Thundertree background also decided to be a paladin of nature, so to make the Thundertree encounter more interesting, when they got there Venomfang stepped right out into the open with dragon cultists around him, and told the PCs he was in Thundertree to clean it up and start a new town where humans, beasts, and nature could all live together in harmony. Reidoth was with Venomfang, having been brainwashed into the cult. The PCs fell for it (no rolls needed), especially when Venomfang offered to pay them to help rid the village of ash zombies. The PCs helped kill the ash zombies and Venomfang paid them in gold for their work. As they were sleeping that night they were whisked off to Barovia (Curse of Strahd). They have now returned from Barovia, and have discovered that Venomfang might be running some sort of con, so the PC with the Thundertree background wants to go back to confront Venomfang. Just one complication, upon returning to Faerun, the PC has also discovered that his parents have moved to Thundertree to live with Venomfang and the other "cult" members in New Thundertree.