You carefully fill the bottle with enough of the cauldron's ichors for what you consider to be suitably sized sample.
With the poison in hand, it is time to return to High Priestess MacDonnell at Chillwind Camp to complete the mission.
Ah, there is plenty in here for study. Well done, <name>!
For now, the next step is in our hands. We'll need to produce a list of reagents that, when combined with what is already in the cauldron, will act as its counter.
The only way we will be able to introduce this is by placing the reagents into the cauldron directly. Alchemist Arbington will have more information on that, so be sure to speak with him at your earliest convenience.