The cauldron is occupied by Roderick McPorcelain.
You can ask him nicely but he is quite rude towards you. Talk to him until you get a note in the journal about the cauldron. Use all the dialoque options.
This is the low-spoiler guide for "Freeing the cauldron" puzzle in Plot of the Druid. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.