You were able to enter the Untamed Thicket and followed the trail of corruption to the Corrupted Spawning Grounds.
It was there that you found a beast during its gestation stage but you were unable to kill it before it could escape and it now roams free.
You will need to head back to Nafain, who continues to suffer as he bleeds out while remaining bound to a tree, causing the corruption.
How To Get Stemming the Flow Quest?
Stemming the Flow is obtained after you have completed the Fangs of Corruption quest, where you attempted but failed to kill the Amalgam of Rage.
Stemming the Flow Quest Objectives
Return to Nafain
After failing to kill the Amalgam of Rage, you will need to take care of the corruption that continues to flow through the land and head back to Nafain.
Speak with him once you reach his location and this leads to a cutscene that ends with you putting Nafain out of his misery by impaling him.
During the cutscene, it is revealed that the beast was born of his blood and that he has revealed the location of Astaroth, which is in Eldhaime.
Stemming the Flow Rewards
After speaking to Nafain and finishing him off, the Stemming the Flow quest will be complete and you will be rewarded with an amount of XP and Gold that scales with your level.
Verdict
It appears that Lilith needed Nafain’s blood to spawn a creature known as the Amalgam of Rage, which is the beast ridden by Astaroth.
She now makes her way to Eldhaime, where Astaroth lies captive and most likely seeks to free the demon and bring him into Sanctuary.
While Fiorban lies next to his dead master, you will need to rush back to Eldhaime to alert the others about what Lilith is planning before it is too late.
