This elder eater of brains is not a giant brain in a stationary pool of salty liquid. It's an older, bigger, smarter, tougher monster that will eat younger jangboko if it feels like it.
This thing will not be found on its own, as it tends to keep a hive of jangboko under its direct control, along with anything it or its underlings have enslaved and enthralled. A jangboko only gets this old and powerful if it's exceptionally smart and an expert at staying alive.