Obviously, the sudden change in the atmosphere is due to both the Black Earl and Vay.
However, it's not just the Black Earl and Vay who are making the atmosphere heavier.
Dox and Kaier also have a part in it, even though they don't fear the stench, they definitely don't want to wander through the sewers.
Given a choice, who would want to go for a swim in the sewer?
Moreover, the situation in the sewer is quite unclear, it's filled with the Dark Aura that the Ghosteye Witch absorbed earlier, and for thousands of years, these dark energies have nurtured countless Magical Creatures.
And the Magical Creature ecosystem in the sewer abides by the Law of the Strong.
Weak Magical Creatures, like mutated Carrion Squirrels, have to escape the sewer to avoid extinction.
