You fill the bottle with enough of the cauldron's poisonous concoction for a suitable sample to be studied.
With the poison in hand, it is time to return to Shadow Priestess Vandis at the Bulwark to complete the mission.
Well done <name> - it should take Apothecary Dithers no time at all to analyze this and devise a counter agent to introduce into the cauldron. If we are able to pump out enough of it into the air, we might be able to weaken all the Scourge present here.
This will no doubt be an ongoing ordeal; we will need to continually hit these cauldrons and introduce our reagents into their mix. Still, if we can keep it up, we will have a huge advantage over Scourge when the time comes for battle!