The key taken from Cauldron Lord Bilemaw is a perfect fit in unlocking the access panel, though it disappears after you use it.  Still, you think you can open the panel now without a key.  A foul stench emanates from the opened panel as an unknown mix of concentrated substances swirl inside the cauldron.

There is a small spigot that the bottle fits well enough under, allowing you to draw a sample for Shadow Priestess Vandis back at the Bulwark. 