Shambling Mound Venomfang

I'm trying to change Thundertree and I'm unsure about a few things.

The idea is to turn Thundertree (the home village of 2 of the PCs) into a place that was attacked and destroyed by Ash Zombies, Awakened Shrubs, and the boss monster Shambling Mound

This happens because the dragon cult got their hands on a divine tome, and accidentally (?) turned part of the population into monsters

Here the things I'm unsure about:

• Why would the dragon cult do this? Are they even a dragon cult? Maybe they could be tied to Black Spider

• Reidoth would help evacuate some of the villagers, but that would imply that this is happening (almost) immediately as the party's arrival in the village

• What would Reidoth do? Should I make it so he has to follow the survivors so that he's not there to make everything easier for the party?

• If I do this, the party' objective should be to kill the monsters and/or the cultists, and get the book to bring it to Phanladin (story reasons that I'm not explaining here, but if you ask me I'll say it). Should they be interested in turning everyone back to normal? If so, I think Reidoth or Garaele should be able to do it with the book

• Should I change other things about Thundertree? Like items locations, encounters location, etc

Thank you for the help! If you want more info on the campaign and/or on the party, feel free to ask!