At this point, the group is looking to start a fight with the Druid Council. There are three key locations:
the Temple of Ehlonna (scene 6a)
Freehaven Library (scene 6b)
City Hall (scene 6c)
I recommend against offering these locations to the players -- let them find the locations themselves through investigation. (On the other hand, it's good to be generous about handing out this information if the players ask!)
It will be necessary for the group to run through City Hall to complete the adventure; the other locations are optional, but may make City Hall easier.
All the locations in Scene 6 contain enslaved werewolves. These poor souls are much like Ambassador Alana in Scene 5a: civilians, kidnapped by the druids, infected with lycanthropy, fitted with silver necklaces, compelled to obey.
Werewolves in human form generally have ragged beards, poor personal hygiene, and no weapons or armor. They're very limited in what they can say. They're allowed to take on a hybrid werewolf form in self-defense, or in defense of the druids or their property. Each has a silver necklace which they cannot remove; if someone removes the necklace, the werewolf shifts to wolf form and will try to flee the city.
(From a utilitarian perspective, it's actually very bad to have werewolves in the forest, because they'll come back and raid the town and eat people. It does get them out of the way for this adventure, though.)
Each of the three druids owns a commanding stone of Malanel, which allows them to telepathically give orders to any or all of the werewolves. The orders can be countermanded by anyone else with a commanding stone; the silver altar of Malanel, under the Temple of Ehlonna, is the root of this compulsion and can override the commanding stones.
A good path to victory in this scenario is to contest the druids' control of the werewolves, either by defeating a druid and taking a commanding stone, or by seizing (or wrecking) the altar itself.
A tolerable path to victory is to burn down Freehaven Library with most of the werewolves in it, and then raid City Hall mostly unopposed.
The worst solution is to blindly raid City Hall. If the group attempts this, they're likely to be chased by Erik the Bear and his werewolves; they might be able to evade pursuit due to the heavy rain, but if they insist on a battle, their chances of survival are pretty low.
The book says it takes an action for werewolves to shift forms, but that's dumb. Werewolves in this module can shift as a bonus action.
Scene 6a: the Temple of Ehlonna
The Temple of Ehlonna is a stone building; there's a big firepit in front of it, but with the storm there's no fire here. On the top floor there is only one guard, named David. David used to be an acolyte of Ehlonna, but now he's a werewolf, controlled by the silver necklace of Malanel he wears. Right now he's forbidden to say anything except that the Temple is closed and Natasha is busy. If the group tries to push past him or attack him, he turns into a werewolf and fights.
The Temple of Ehlonna is warm and should feel welcoming, but the terrible evil performed in the basement has given a psychic chill to the whole building. Anyone who can sense evil (for example a paladin with divine sense) feels that this place is tainted. Someone who can't sense evil might just feel vaguely uncomfortable here. Investigating, the group might discover the trapdoor, behind the Altar of Ehlonna, which leads to the basement. In the basement is the Silver Altar of Malanel, guarded by Natasha Stormcaller and two werewolves. Natasha was performing a ritual to maintain control over the werewolves; if attacked, she shoots lightning from her storm staff and orders her werewolves to attack.
If the group defeats Natasha, they take possession of her storm staff and her stone of werewolf commanding, though they may not recognize how to use either. They also get access to the silver altar of Malanel, which radiates evil; they might destroy it (freeing all the werewolves from compulsion, in which case they mostly flee the city) or they might attempt to control it, letting them give orders to the werewolves.
Using the altar requires a ritual sacrifice to Malanel. The sacrifice will not involve killing; instead, the character must force a creature to do something it does not want to do, while it is in contact with the altar. Threats and violence and intimidation are fair game; persuasion and promises are not. Any creature, and any act of coercion, will suffice to gain access to the altar. The characters may learn about this ritual by making a religion check or by reading a nearby prayer book.
Scene 6b: Freehaven Library
Freehaven Library is occupied by the druid Erik the Bear and several werewolves under his command. They've been waiting in the library for the past several hours. They don't smell very good and they're scaring the civilians; all the normal library guests have left, and the librarian is trying to think of an excuse to leave as well. Erik doesn't care.
When the group gets to this point, the first question they ask is usually: "hey, can we set this building on fire?" Freehaven Library is made of wood, and very flammable. It has one door, and it has two windows which are large enough for a werewolf to leap through. If the group wants to kill the werewolves by setting the building on fire, they will need to barricade the door and both windows for several rounds of combat. If they fail to barricade something, a werewolf (possibly a badly burned werewolf) gets out of the library and has to be fought.
The group received potent flasks of alchemist's fire from the mayor, back in Scene 1. One of these flasks is sufficient to burn the library down, if thrown into the interior of the library. In the raging storm, nobody comes to defend the library or join the battle.
Erik the Bear can, true to his name, use a shapeshifting power to become a bear. If the bear is defeated, it turns back into Erik -- he does not lose his own hit points when the bear is hurt. If the group defeats Erik, they find his commanding stone of Malanel (which he uses to control werewolves). They also find his bearskin cloak, which gives a druid wearer the ability to wild shape into a much larger bear than normal. Erik can wild shape as a bonus action.
Scene 6c: City Hall
City Hall has two floors. It can be entered from the front door, the back door, a trapdoor on the roof, or by breaking into a window.
The back door is locked and barred, and would be difficult to open. The front door is locked but not barred, and a druidic glyph on the front door causes it to shout "alarm! intruders!" when opened without disarming the glyph. The windows and the roof trapdoor are locked but not barred or alarmed. Remember that the storm means adventurers are unlikely to be spotted, but also might have difficulty climbing the building.
If the group opens the front door and it sounds the alarm, Erik the Bearand his werewolves leave the library and attack. This is a lot of werewolves and it should be clear to the group that fighting them in direct combat would be unwise. (The heavy rain hinders pursuit, so the group can probably run away if they decide to.)
Mayor Boris's office is in the center of the lower level of City Hall. It's easy to find -- an ornate nameplate on the door says MAYOR BORIS in big letters. The door has a druidic fire trap glyph on it; trying to open the door triggers the trap, dealing fire damage to everyone nearby. The druidic fire glyph has ten charges per day.
Mayor Boris is in his office, guarded by two werewolves. (If the group has been foolish and they're being chased by werewolves from the library, you probably should omit these werewolves to avoid a TPK.) Mayor Boris fights using woodcraft: he can cause brambles to spring up from the wooden floor, ensnaring and injuring individual opponents.
Mayor Boris's desk is guarded by a druidic fire glyph much like the one on the door. In the desk are some papers detailing his plans (see Scene 0: The Druids' Plan) and a wand that can create fire glyph traps. There's also a key which unlocks the treasure chest below the trapdoor.
Under a carpet in Mayor Boris's office is a trapdoor. On the trapdoor is a druidic fire glyph much like the ones on the door and the desk. (Surely by now the players have figured out that everything here is trapped, and they've found a way to find and disarm these glyphs?)
Under the trapdoor is a small cubby containing a lead case with runes on it. These are runes of binding and containment; in the lead case is the Orb of Storms.
If the group opens the case to check that the Orb is there, this unleashes its power. The storm raging overhead gets much worse, with lightning and hailstones. Traveling back to Cedar Valley will be very difficult in the hailstorm; the group will need to seal the Orb back in its case, and then find some way to reduce the power of the storm. Perhaps a good Nature check might allow a character to convince the Orb that they're allies; perhaps an Arcana check might enhance the runes of binding on the case, weakening the Orb's connection to the storm.
It's possible to use the Orb against the town. If the group has barricaded City Hall and Erik's werewolves are still outside, the hailstorm will gradually wear them down, and it might make them seek cover. In desperation, a member of the group might attempt to use the Orb to channel lightning attacks; these should be devastatingly effective if the character makes the appropriate skill check (I usually ask for a Charisma save to enforce your will upon the Orb). If the check is failed, the Orb should damage the party instead. Note that the Orb is sentient, and it speaks telepathically to any character that touches it -- but it's very angry to find itself in a town, and its goal is to destroy the town and kill everyone in it, returning the area to its original state of nature.
Also under Mayor Boris's trapdoor is a treasure chest -- like, an actual chest with a lock. This chest is trapped with a druidic fire glyph, but can be opened with a key in Mayor Boris's desk. Smashing the chest also works. In the chest is a lot of gold, perhaps two thousand or so gold pieces.