I was stuck on this as well, and finally figured out something that is…less than obvious - I am going to spoiler it, but its not a spoiler per se, but more of a “Duh” moment
You need to go to the crypt in Hunters Folly, and use your Word of power on the door there (Duh!) and in there is an elemental rod, which in my my case was the last one missing at Agathas Altar. That leads to the series of small quests in the Abbey that expands on both the area, but also gives you access to the cauldron.
I too am 25 hours or so, and just figured this out last night - its a bit obtuse though. But let me say this - All elemental rods are there, avaialble in the first area.