How to get this quest
After completing What Lurks Within The Mountains and dealing with the bandits on the Lumeria-Union bridge (see Eternal Day Of Sloth), you find this quest in Trademond's guild.
Going to the swamp
First, talk to Trademond's alchemist. You will give access to the swamp area north close to the Lumeria-Union bridge. Once you enter the swamp, Desmond becomes a partner for the quest.
The four plants
Walking and fightning in the swamp leads to take damage over time. Four coloured flowers are spread through the area. Interacting with one of those flowers leads to a fight against a mantrap miniboss accompanied by two scorpion bees. Each won battle against a mantrap leads to reduced environmental damage. Defeating all mantraps completely clears the poison in the air.
Defeating all mantraps also weakens the main boss northeast of the swamp.
Completing the quest
Your objective is to defeat the mutated serpent. But before you fight it, you want to interact with the alchemist's apprentice close to the boss. You can get a Vice option (10 Vice requirement) to unlock a bonus reward later.
The mutated serpent is weak to the Light and Fire elements. Its dangerous skills are locked out if the mantraps have been defeated. Desmond's base ATK is underwhelming against the serpent, but his passive skills can make him useful. The serpent is finally immune to poison and can paralyze you with its basic attack after uncoiling. This makes good offense important to win quickly before being worn out.
After defeating the serpent, go report to the alchemist. This upgrades the recipe for poison coating. Furthermore, if you took the prior Vice option, the alchemist unlocks Demonbane.
Complete the quest by reporting to the guild clerk.