From as early as Chapter Two, your attention will be pulled in other directions thanks to the flurry of side quests you'll gather up on your to-do list throughout the campaign. One such quest you'll likely trigger this chapter is The Emissary.
You'll come across the emperor's seal, but where it came from, and who was travelling with it remains a mystery. Here's how to make sense of this early area, and how to navigate the poison fog.
Finding the Emissary's Seal

When you get to the 'The Other Side' Lantern, after clearing the corruption on the bridge, you'll be able to push forward, or you can go down a little side road on the left, which is covered in a thick cloud of green poison.
You can make your way through the initial parts of the fog, where you'll find a train carriage with the Emperor's Seal inside, beginning the quest and raising the question: just what happened here.
However, aside from pushing through a small patch of poison to get some Standard Firearm Parts, you won't be able to get through the poison right now.
How To Get Through The Poison Fog In Valor Mortis

To get through the poison fog, you need access to a particular Transmutation: the Shield Transmutation.
This is acquired rather late in the game during Chapter Ten, The Castle. You'll need to fully explore the place, and in doing so, you'll be able to acquire this power on the top floor of the Castle.
Then, you can use this power to get through poison patches, as when this skill is held, you cannot be harmed by any status effects. Plus, it's a pretty handy defensive strategy in battle too.
Fighting The Emissary

Once you have this power, you can make your way back to this early-game area and walk through the fog. Here you will encounter a mini-boss who might catch you off guard.
They seem like a pretty standard human enemy, but their gimmick is that they have the ability to spawn an entire firing squad who take shots at you in unison. It's no exaggeration to say that if you take the full squad's bullets head-on, it's almost a guaranteed one-shot kill.
We would suggest dropping back and using your Musket to land a few strong headshots from cover. Then, when he inevitably runs at you to close the gap, we would suggest switching to your Blunderbuss, emptying the barrel, and then fighting the rest of the fight at close quarters with some melee and parry action.

Defeat this mini-boss and explore the immediate area and you'll find a piece of fine cloth on a spike. Investigating this will round off the quest, but you'll also be able to get your hands on a new Amber, the Perfect Copy Amber, which is found in a chest nearby.