Beyond the prison chamber is an exit to a snowy mountain pass which is heavily patrolled by the javelin dudes followed by wolves and the great Nords. In traditional Dark Souls fashion, since I was terribly outnumbered here (worse than 10:1) I decided to go to the old staple: Panic Sprint through everything and somehow it worked!
After opening a few shortcuts I found myself entering a large, smelly chamber full of carcasses - a hive of giant flies? While very yucky, these weren't that dangerous to get through since I wasn't silly enough to wake all of them up at the same time. A nearby lever opened a secret passage back at Sister Friede's chapel. How I saw what happened in a cut scene far away - I don't know, but that's not the first time it's happened either. :P
A secret passage from the hive then led to a tangle of roots down to a familiar looking circular tower... damnit. This IS the same painting that I was in before! With nowhere else to go I used a homeward bone to return to Friede and get some answers.
She just casually implied I should leave but didn't actually stop me from descending the secret steps into the large, underground sanctum. At the end was a giant bird man, peering into a golden cauldron. It turns out this is the titular Ariandel.