Available in the Adventurer's guild after saving unlocking Demonic Vault.
Go to Riverflow and talk to John. Next, examine the two water buckets. One is close to John, the other is close to the river. Bring the two water samples to Trademond's alchemist to unlock the next part of the quest.
You can now take the path in Riverflow leading to the forest. This is where you'll find a boss battle against two mutated hydrangeas.
If you don't have a wind spell, the fog will make you blind and you won't be able to dispel it. Nadia can teach you Tailwind in the Boar Hut to solve this issue. Furthermore, the mutated hydrangeas can reapply the fog during the battle, meaning you must keep enough MP for this possibility. Note that if you win the battle while the fog is present, you'll get offered a lewd scene with John, but this scene doesn't bring any reward.
The other issue of the fight is finding a way to damage the bosses effectively. Indeed, John's normal damage output is weak in this fight, the bosses can debuff the party's ATK and can regenerate. Here are some options to win the fight without removing Aura's collar:
After defeating the mutated hydrangeas, report to the guild to complete the quest.