1 -Jenniker's Corpse
2 - Mana Pendulum
3 - Belt of Fluidity
4 - Gem Lattice
5 - Lever
6 - Artifact
Right after you enter the cave, kill the first wave of enemies and examine the nearby corpse. You will find a journal, which describes three artifacts necessary to neutralize the poison. While following the tunnel, you need to avoid the plants that attack you and dash across the water, as quickly as possible, because it is poisonous.
You should also remember about using lever to open passages.
Mana Pendulum is in the chest to the South and it is guarded by a group of enemies, led by two Cave Banshees. Try to kill the biggest enemy as quickly as possible, because it will keep summoning reinforcements into the battlefield.
After the battle, pick up the first artifact and collect the Belt of Fluidity hidden in the chest to the West.
The third item (Gem Lattice) is being guarded by a group of creatures, like earilier, but this time, you will find it in the Northern part of the dungeon.
Collect all the ingredients and return to the corpse that you examined earlier. Right next to the corpse, there is a broken artifact, which you need to use. Then, 4 options will appear.
If you deactivate the device, the river will be clean again and the Ronds will get better. If you select the "Sanguine" option, you will improve on the condition of the couple that send you here, but you will have to return to the dungeon and deactivate the machine anyways.
"Phlegmatic" ends up in the death of the couple but, only deactivating the artifact ends the "Melancholic" quest. In any case, you will receive gold as a reward.