Online:A Poisoned Heart

Quick Walkthrough
# Find the Strange Cloud of gas just south of the Hatching Pools Wayshrine.
# Talk to Marius.
# Find a wasp husk.
# Talk to Rhea at the Imperial camp.
# Collect the poison gas.
# Give the poison to the Imperials.

Detailed Walkthrough
The quest begins when you stumble into a "Strange Cloud" just south of the Hatching Pools Wayshrine. Your character will quickly choke, vomit, and fall unconscious, with the screen fading to black. They'll awaken in front of Marius and Rhea. Speak to Marius:

:''"You're very lucky. Rhea risked her life by dragging you out of that poison gas.
:Most Imperials aren't so partial to members of the Pact, but she's naive."
:;What are Imperials doing in Shadowfen?
:"Cleaning up a mess. I bear no responsibility for it, mind you.
:If Rhea and her diplomat father hadn't created such an uproar about poison harming backwards swamp-dwellers, I wouldn't be here in the first place."
:;What do you mean?
:"Due to a clerical error, the Empire sent poison downriver in a trash shipment. Certain people feel we are responsible.
:Sloshing about in the muck is beneath me, but you don't seem to mind. How about it? Solid Imperial coin for cleaning up this mess?"''
:;I'll do what I can. What do you need?
:''"Rhea needs the husk of a giant wasp for some sort of container. I'm sure she'd be happy to tell you all about it. Just bring the husk back to our camp, so we can get on with this."

Kill a Giant Wasp in the indicated area, and collect a wasp husk from its corpse. Then, find Rhea and Marius at their temporary campsite, located about halfway between Deep Graves and Murkwater. When you arrive, Marius will grouse, "Well, that took you long enough. Did you get stuck in the mud?"

Speak to Rhea:

:"Don't mind Marius. He was ordered here to look after the interests of the Empire. But really, all he cares for is his own advancement."''
:;Here's the husk.
:"  give me a moment and … done."
:;What is done?
:''"I've enchanted this container to draw poison within it. Place it in the path of the roaming poison. Once the container collects the poison, you can safely recover it.
:We need to hurry; I hope no one else has been hurt."
:;Marius only asked me to find the wasp husk.
:"Oh? I assumed you were part of the clean-up. What's Marius planning?
:I don't know you well enough to ask for a favor, but I implore you to listen. I fear what Marius plans for the poison."
:;What are you asking?
:"Once you've collected the poison, bring it to me. I ...
:It's Marius, he wants to speak with you. Please, tell him nothing of what I said!"

Marius has been standing almost directly next to Rhea for this entire conversation, well within earshot and perfectly capable of hearing everything she just said. Speak to him:

:"You have a container, good. Are you ready for what's next?"
:;Ready.
:"Rhea and I will take care of the poison in this region. You handle everything past Hissmir.
:Once you've collected the poison, bring it to me at the disposal site near Zuuk."''

If you have the Persuasive Will skill, you can choose to persuade him:

:;[Persuade] Rhea doesn't trust you. If you want my help, I want to know why. 

Alternatively, if you don't have the skill or are worried about ratting Rhea out, you can "agree" to help him:

:;I'll bring the poison to the disposal site.

You will need to go to two locations north of Hissmir and intercept moving clouds of poison. Do this by standing ahead of the cloud's path of movement, aiming at it, and interacting with it from a distance, in order to place the enchanted container on the ground directly in front of you. Once the container is placed, wait for the gas to hit it, and when an objective marker appears inside of it, interact with it to collect the container and the gas.

If you place the container incorrectly, you'll need to wait a fairly long time for it to disappear before you can try again. If you're interrupted while placing the container (e.g. because you move), you will likewise have to wait out a long cooldown. There are two factors that make this even worse. First, stepping into the cloud at any point will cause your character to choke and vomit, and decrease your movement speed. Second, one of the poison clouds is surrounded by a giant snake, two crocodiles, and other creatures -- and their respawn times can be quick.

At any rate, once you have the gas, you'll have the option to deliver it to Marius or Rhea. You'll find them both at the same dig site.

Giving the gas to Marius
Speak to Marius:

:"Do you have the poison gas? Quickly now, give it to me."
:;Here you go.
:''"This is the last of it, then. Anyone who tries to stand in my way will regret it. Here's your coin, as promised. Now begone, there's much work to be done."

After the conversation finishes, Marius will throw the gas at Rhea, killing her instantly, and yell out:

:Marius: "To Oblivion with your interference. For the Empire!"''

Marius will run away, leaving Rhea's corpse next to the grave he was digging.

Giving the gas to Rhea
Speak to Rhea to complete the quest:

:''"Marius has grown very impatient. I'm almost certain he'll use the poison for personal gain. It can't fall into his hands."
:;Here you go.
:"I'll get it as far away from Marius as possible. You've saved numerous lives today."

After the conversation finishes, Rhea will trap Marius with a spell and the two will speak to each other:

:Rhea: "Marius! I'm onto your treachery. You won't get this poison!"
:Marius: "If you set foot in Cyrodiil again, you're dead. Do you hear me, Rhea? Dead!"''

Rhea will walk away, leaving Marius trapped there.

  

Quest Stages